My view: It's "theft" of potential profits, not profits that currently exist.
The justification is similar to anything that infringes on your ability to do business. If someone's say, scaring customers out of your store, you're pretty justified in claiming they're hurting your sales, even if the other customers never actually paid any money.
The problem with protection of potential stuff is well, it's potential. Not everyone who pirates a game or song would've bought said game or song if piracy was not an option (I'd wager a guess that very few would, actually). Plus, even if you accept the protection of potential things as valid, you have to draw the line somewhere; competition, for example, quite obviously cuts into potential sales as you can't set the price to be whatever you want. However, competition is justified despite that. What else is justified, and what is not?
Further note: Several markets simply would not exist without this sort of protection. Anything that can be turned into data (music, games, etc) are intellectual property; without protection of that, music, art, games, would all be hobbies rather than jobs. Only being able to make money off of physical objects seems kind of silly to me. I dunno about you, but I'd like to live as an artist selling my art, not by selling random doo-dads like t-shirts and novelty mugs.