I know you didn't mention Princess but you really should consider them. They are the experts when it comes to Alaska. Among other things, they go to Glacier Bay National park, which Royal Caribbean does not.
What makes you think that there is no reason for it? Just because you don't know the reason doesn't mean there isn't one. I'm sure all of the maitre d's on all of the ships didn't just randomly decide to implement this on their own.
It doesn't sound like the issue is whether or not to tip the porter. It sounds like the issue is that the supervisor threatened that the luggage would not be delivered if he wasn't tipped.
I would not expect Royal Caribbean to find out about this or do anything about it because of you posting it here, if that is what you are hoping. You might consider notifying Royal Caribbean directly instead of notifying someone on the ship.
Did you notify anyone at the port or on the ship about this? What did they say?
Also you might consider rewording the end of your last sentence. It doesn't make a lot of sense.
I agree that they can be kicked off at the next port, but can they really be prosecuted by the local police for something that was done (most likely) in international waters, or in the waters of another country?