There is no such regulations in the US DOT policies. If denied boarding due to oversales there are mandatory compensation.

When you say improper reason, that likely means there is some dispute over the reason -- such visa guidelines interpretation? There areno proscribed regulations on compensation in those cases, but you can request some, if you can convince the airline it was actually improper -- the specific issue matters.

Following on the prior poster, what was the motivation for this query?