I don't consider $1 per bag a rip-off at all. I would still tip.
And furthermore if everyone gave a generous tip they would not need to charge extra for the bags.
If you can afford to cruise you can afford an extra couple of bucks for the bags.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Independence of the Seas 12/29/2013 Top-notch!