if they control your computer, they can prevent you from incurring in 'illegal' activities such as piracy
but it all boils down to the logic of the market, the raw fact that capitalism works even with marginal costs. but when copying (and distribution) costs go lower (less than 'marginal' down to zero cost) it all starts to break down
if people aren't selling digital assets to each other (which doesn't make sense with the technology we have right now), they cannot be taxed and so on.
solution: fix the technology. make it so that only those with specially authorized keys (trust worthy actors) can copy digital information at will. everybody else will have to pay them for this privilege.
oh and nevermind the fact that computers work by copying bits all over the place