If that's the case, why isn't Sony suing Studio Canal? The proper license for Sony to get is one which doesn't allow Studio Canal to take it away after sale in any way. So did Studio Canal somehow hack into Sony HQ to violate that license, or did Sony simply screw over its customers by not getting the proper license in the first place?

For Sony, the correct move here would have been to not list Studio Canal titles in the first place, and put out a very public statement saying that they aren't being listed until Studio Canal agrees to make purchased licenses perpetual as they should be.