In theft you take something from someone else, and they are then deprived of what you took. I take your loaf of bread, you have one less loaf of bread.
If I copy your movie, you still have your movie.
Now, if I (somehow) get hold of a copy of a digital file you want to sell me, you'll be less likely to be able to get me to buy it (although, studies have shown that those that copy content a lot, also tend to pay for content a lot).
So we need another word than "theft" - and that word certainly isn't "piracy" - people that violate copyright, rarely murder people or disrupt trade routes and critical infrastructure.
I'm not saying we don't need some system of compensation if we want to continue to produce art - but just as a boycott is a very passive act of resistance, so is not paying for crippled (DRM) content. Unfortunately it doesn't seem that there's a need to prove that someone watching a copy for free would be more likely to pay for that copy than go outside and play a game of catch if the content wasn't available for copying.