I don't recall voluntarily agreeing to copyright laws. Why, then, should I be obligated to obey them? Especially given that it infringes on my right of free speech to be forbidden to copy a work, regardless of whether it originated with me or not.
The answer of course is that majorities tend to rule, and if a majority has a legitimate power to establish a copyright law, they likely have a legitimate power to establish an antitrust law.
Oh, and BTW, I think you may have misunderstood what an ad hominem argument is. When someone says that you're stupid, and that therefore because you are stupid, your argument is wrong, that is an ad hominem argument. It's fallacious because even stupid people can make valid arguments; out of the mouths of babes and all that. OTOH, when someone says that your argument is wrong, and also that you're stupid (not as a necessary consequence of having made a wrong argument, mind; everyone makes mistakes), that isn't an ad hominem, that's just an insult. Maybe it doesn't do much for the quality of the debate, but it can have a certain rhetorical usefulness, and it can be fun, you dipshit.