More specifically, that whoever was involved in the policy (the military, state department, etc.) was aware that the buffalo was the primary source of food for many western Native American tribes, and so this was an attempt to weaken their by wiping out their food supply.
The argument was basically that there wasn't any other particular reason to incentivize buffalo huntings--they weren't a hazard or a nuisance, settlers had other options for food if necessary, and so on. So it had to have been first and foremost a deliberate, calculated ploy to thin out by attrition the Indian populations that were harrying westward expansion of the United States' frontier.
Was this unsubstantiated or do we have any evidence for it?
First off, I'd like to start out by saying that people did indeed purposely wipe out buffalo herds in order to deprive Indians of their primary resource. It was very successful at driving out almost every Indian tribe out of Texas and into Oklahoma sans the Comanche.
The United States specifically did not enact any "buffalo killing" policies, but instead looked the other way even though some conservationists were concerned about the buffalo being wiped out completely. In the 1880s 4-5 million buffalo were killed.
There was also an economic reason behind this: Buffalo hides became increasingly popular when railroads were being built throughout the midwest. The railroads made it very easy to transport multiple hides across the country, so it drove merchants to kill more buffalo. One of the most famous buffalo killers was "Buffalo Bill" who reportedly killed 15-20 buffalo a day by himself.
One of the more interesting stories associated with buffalo killings was that when people took trains touring the country they were all given guns to shoot at the buffalo they passed by.
so tl;dr: yes people killed a LOT of buffalo and while it was originally to get rid of the Indian's primary source of food/material, it was also economic.