Or they thought they could cope with the click fraud. They are big enough to know all about the patterns of impressions/clicks on any site or type of site.
IMO, they underestimated the amount of preparedness they needed to cope with click fraud. This is surprising to me, since these measures are well known and understood within the security community at large.
I sometimes wonder whether Google isn't using Adsense to get a list of the honest webmasters and a blacklist of those who can't be trusted.
If this is true, then it is a rather crude way of doing so.