How come one is able to pick release even if the order is on hold ?
It depends on type of hold.if it is manual hold at Order Header level/Order Line Level,you will not be able to pick release the order unless until you release the hold manually and proceed ahead.
If it is the credit check rule based hold defined at the OM transaction type level ,then check out at what level is the hold defined ie. at picking/shipping/booking. If the hold exits at shipping level it seems and that's why it allows to pick inspite of hold