I suspect this boils down to a cost/benefit tradeoff.

The cost of strictly enforcing stated rules for ads is felt immediately and fully by Google. It is concrete, measurable, and probably non-insignificant; the benefit is essentially goodwill from the narrow sliver of the web that is verbal about this. That is intangible, difficult to measure, even harder to assign a reasonable dollar value to (meaning the ROI could look negative), and the benefit would accrue to parties other than Google who might not even notice.