Actually, we don't - there is no legal/treaty reason why a government could not offer a negative corporate income tax rate (at least in the lowest N-1 tax brackets), and we may actually see that if corporate abuse of political processes is allowed to continue far enough.
No, I'm saying that "this value is not allowed to go below X" is different from "the lowest current value happens to be X" and we have the latter, not the former.
