In the first case they're held accountable because business interests are compromised. In the second, no business interest is compromised by screwing up Joe Pu

In the first case they're held accountable because business interests are compromised. In the second, no business interest is compromised by screwing up Joe Public's computer (except Joe's, but he has no material social leverage, so...). Thus no accountability for ad networks, even if there logically and legally should be.