It's not stealing, it's unauthorised redistribution. They don't lose anything from piracy, except for potential customers, which is a pretty intangiable concept. Don't think it's even remotely as bad as going into a tech store and taking all their DVR's, or even literal piracy by chasing down another boat with guns and taking their cargo.
System wide ad blockers can't block a lot of ads, namely same-domain ads or those that are built into the html. Much rarer than the external page kind (DNS ones) thankfully.