Because people will camp prisons because people will be guaranteed thrown there and will fly out.
No such guarantees can be made about other places in galaxy.
People coming out of prison will already faced their penalty, having guaranteed PvP attack when coming out of prison would be just a bit too excessive.
The very people that pick on less well armed craft than them get special protection by being taken out of open? That doesn't seem right to me. If their enemies are camped there waiting, let them face up to them.
...or mode switch like the cowardly pirate dogs they are.