Well, for one, the threat of the FCC, FTC, or FDA fining them enormous sums of money. Google is damned if they do, and damned if they don't. If they allow any and all ads, even from those who are outright running congames and ripping people off, they will be accused of profiting off of immoral activity (as they were with the drugs-over-the-internet case.) However, if they then turn around and refuse to run such ads, they are accused of censorship, and once again, being evil.

They have to walk a tight rope, doing enough curation to avoid government and public outcry, and at the same time, not doing too much. The world isn't black and white, it's always gray, and deciding where to draw the line is not something which is objective.