Honestly, the answer to this question really is, in a way, "because racism."

If people disliked black people, then shouldn't they be glad, or at least not averse to seeing black soldiers charge into the breach and get killed?

Not exactly. The common theme throughout American military history when looking at reactions to black troops tends to be that most whites did not want blacks to have military training or to become familiar with weaponry (black troops were often relegated to supply/other non-combat positions). More broadly, whites detested the notion that blacks might gain some measure of pride from military service and demand something better than the degradation to which most of them were subjected throughout slavery and Jim Crow. Southern whites especially thought that allowing blacks to serve in combat units would lead them to think that they were on an equal footing with whites, which was totally unacceptable. Over and over again, you see whites reacting badly to black troops because they feared those blacks might come away with "uppity" ideas. There are cases of racist whites attacking and in some cases lynching black troops for this very reason. During the Civil War, Confederate leaders tended to be totally inflexible on refusing to arm blacks, since they knew that if blacks made good soldiers, the Southern way of life, which was based on slavery, would be discredited. As Georgia governor Joseph E. Brown put it, "Whenever we establish the fact that they are a military race, we destroy our whole theory that they are unfit to be free." American racial segregation was based on keeping blacks in a state of permanent subservience, so this kind of thinking is not surprising.

historically the same old arguments were trotted out each time to combat desegregation or a larger black presence in the military. They had poor eyesight, they were lazy, they were cowardly, and so on. Yet the historical record clearly proved otherwise with these highly decorated units. What was people's response to that?

I consider these arguments a means to an end, the end being "keep blacks from serving in combat units." Racism is not rational, so it's unsurprising to see irrational arguments deployed over and over again. Interestingly, during the Civil War, you do see a major shift in how people thought of black troops. While some Union commanders (and many southern whites) did not think blacks could make decent troops, many had changed their minds by the end of the war. Union Gen. George H. Thomas, who grew up in a prominent Virginian plantation family, won a decisive victory over CSA Gen. John Bell Hood at the Battle of Nashville in December 1864. Many of Thomas' troops were black, and upon seeing how well they performed, he pronounced his doubts on their abilities settled, stating that "Negroes will fight." You see the same kind of shift happen amongst the Union rank-and-file after a series of battles in which black troops distinguished themselves (the Battles of Milliken's Bend, Fort Hudson, and Fort Wagner). One particularly famous quote comes from an anonymous Irish soldier: "I never cared much for Negroes before, but Jasus, they are hell for fighting." But this respect did not translate into any kind of long term benefits for blacks. After the Civil War, the North eventually gave up on protecting freed Southern blacks, abandoning Reconstruction in 1877. The same phenomenon took place during and after both world wars - black troops distinguished themselves and got little in return. Ironically, Southern whites' fears about black troops were not baseless. Many civil rights leaders in the 1940s had served in WWII, and were not scared of the Klan, having faced the Wehrmacht or Imperial Japanese Army.

Sources:

Glatthaar, Joseph T. "Black Glory: The African-American Role in Union Victory." In Why The Confederacy Lost edited by Gabor S. Boritt, 133-162. Oxford University Press, 1993.

Farber, David and Bailey, Beth. The First Strange Place: Race and Sex in World War II Hawaii. Johns Hopkins, 1994.

Keene, Jennifer. Doughboys, the Great War, and the Remaking of America. Johns Hopkins, 2003.

Payne, Charles M. I've Got The Light of Freedom: The Organizing Tradition and the Mississippi Freedom Struggle. University of California Press, 2007.

The record of black soldiers prior to desegregation wasn't QUITE as universally good as you suggest. In World War I, the 92nd Division was not particularly impressive, though there were a lot of reasons for this, including poor handling and inadequate training as a unit by white superior officers.

By contrast, the 93rd Division fought under French command, with its component units made constituent parts of larger French units. French officers were less inclined to automatically assume black inferiority than American officers. As a result, the division did extremely well and the division's units and individual soldiers were highly decorated by the French.

There's a new book out on the 369th Regiment (the "Harlem Rattlers"), one of the regiments of the 93rd Division, by Jeffrey Sammons and John Morrow: http://www.kansaspress.ku.edu/samhar.html

and typically earned medals of honor at a rate higher than that of non-black soldiers.

I'm not sure where you got this, but this is absolutely not true. As of 2009 there were 3,464 medals of honor awarded, 88 of them were given to 87 different Black soldiers. This is in large part because Black heroism was ignored.

Take Doris Miller. He was a cook with no gunnery training but shot down several Japanese planes and was the first African-American awarded the Navy Cross. When the Navy released names of those receiving commendations for their roles in Pearl Harbor, he was simply 'unnamed Negro.' He was widely celebrated in the Black press and helped rally support for the war amongst African-Americans. But it took a letter writing campaign to get him the Navy Cross.

Info on Doris Miller from Long Memory: The Black Experience in America by Mary Francis Berry and John W. Blassingame