These are not differences in principle. These are just practical differences that affect efficiency of distribution of information, with a little moralistic paint on it.
There's no difference in principle between homosexuality in the UK in the 60s and now. But it was illegal then, not now. Something being legal or illegal is not a fundamental difference. Something being fundamentally different can be a reason something should be legal or illegal, but something being illegal cannot be used as a justification that it should be.
As far as copying, looking at the fundamental principle that libraries operate on, that information should be made available to as many people as possible as best we can regardless of their socioeconomic status, the only difference I can find is that torrenting and copying are more efficient than libraries in achieving the stated goal. When libraries do it inefficiently it is regarded as a virtue, when we do it ourselves more efficiently without institutional support we are regarded as thieves. But fundamentally, are they not the exact same thing?
My question addresses yours because the answer, if it is "they're not fundamentally different in principle" which is my position, negates yours entirely, and if you can show me how they're fundamentally different in principle, negates that very powerful and apt criticism of your position. The answer basically determines if you're right or I'm right.