It's cheaper to write a set of guidelines and not enforce them than to enforce them, which might actually reduce revenue. The incentives for them are such that

It's cheaper to write a set of guidelines and not enforce them than to enforce them, which might actually reduce revenue. The incentives for them are such that it's better for them not to enforce their own guidelines. If Google was getting fined say $1000 per offending ad impression, they'd clean up their act really quickly. As it is currently, they don't have a real incentive to do so.