Who is "the US" and "they"? Scientists knew how dangerous nuclear weapons were, though those working on building the first ones were preoccupied with their goal. When the first successful nuclear bomb went off in July 1945, J Robert Oppenheimer, one of the chief scientists at Los Alamos, said "Behold, I am become death, destroyer of worlds." The magnitude of what he had done finally hit him. And he became anti-nuclear weaponry.

That said, there were always folks who weren't concerned with the dangers of nuclear bombs. Over time, more concern about the dangers was expressed. You may want to watch The Atomic Cafe, a riveting documentary.

They knew even before the Trinity test that nuclear weapons detonations would deposit radioactivity into the atmosphere, especially if detonated in a way that caused the fireball to mix with dirt and debris. They underestimated the amount of fallout produced by thermonuclear weapons until after the Castle Bravo test in 1954. Their understanding of its effects on the ecosystem increased over the course of the period of atmospheric nuclear testing (1945-1962). I think you misunderstand them a bit — they thought that the risks caused by using nuclear weapons to make canals, ports, etc., were worth the benefits, and they knew quite a lot about how much radioactivity would be produced. They had different standards for radioactivity exposure back then than we do now (the amount permissible has been a steadily decreasing thing over time), but they didn't plan any of those things out of ignorance of the fact that nuclear weapons didn't make things radioactive.