"we have prisons
because people ARE responsible
for their behavior"

That is precisely the danger of the free will delusion. It makes people believe that the purpose of criminal justice is to punish rather than to protect society from further criminal behavior. It fosters a punitive "people deserve what they get" mentality which allows us to justify exploitative hierachies and excuse all kinds of abuse and cruelty by saying "they deserved it."

Without the belief in free will, the world would have already eradicated poverty, for example. Poverty is only allowed to exist because the wealthy are able to argue that the poor somehow deserve it due to their own "free choices."

"Free will" is a very dangerous delusion. It is at the root of a great deal of evil. The sooner we realize this, the better.