I have already said i have a HUGE issue with this. It is dishonest of these guys to "pimp" for tips - which is what they do when they say "I'm your porter and I'm making sure your bags get on the ships." No, the ship staff does that. They just get them to the security area.
There used to be info on the web about lawsuits they had about this in Miami, but I can't find it anymore. Apparently, the original dock manager in Miami was quite a well-known local character who got away with all kinds of stuff.
As I recall, they put the same signs (from Ft Lauderdale) up in Miami for a short time, but they were taken down after a union agreement. The agreement said the guys would not solicit tips - as I recall. Well, look what happened.
I have argued with these guys, and I have also tipped them. I dont think it makes any difference. I tipped them in Los Angeles and my garment bag arrived with all the hangers broken (expensive ones) - no one reimbursed me.
I personally would never give them more than a dollar. It's enough that they won't do anything mean to you, and you won't feel ripped off, either.