Yet when you share an mp3 with someone, of which nobody makes any profit, it suddenly becomes a filthy sin.

Under certain circumstances, this is actually illegal even for the people that own the copyrights. It's called dumping and it's a well-established illegal monopolistic practice. The idea is that if company A has a product that company B wants to perform poorly, company B can produce a similar product and sell it under cost or give it away. If company B has more gold in the coffers than company A does, company A could be put out of business. It's essentially a war of attrition, and it's illegal most places I'm aware of, because it allows company B to exert its influence in one area in another unrelated area, which is pretty much the definition of illegal monopolistic practice.

And just because it's not one particular company that dumping doesn't make it less wrong. And, in this case, you're not even dumping a similar product. It's the exact same product.