The IP address is the go to for illegal activity, and your ISP will cut off your internet and issue a cease and desist memo to your account should activity like that happen.
Most providers offer up a few warnings before they close your account perminantly, but it's their prerogative.
The RIAA in particular has had a hard time bringing people to court for things like media sharing with the IP being the deterministic factor, because there is always the claim of innocence ("I was hacked!" or "My router was unsecure!"), which frankly are potentially valid alibis (though many people simply settle when facing litigation and are obviously guilty). So, as it stands right now, when push comes to shove, you can probably prove ignorance and innocence.
However, in the end, you are defrauding a company and stealing a service. Period dot. Can you do it? Sure. Should you? That depends on where your morals lie.