How is that relevant? The government is not obligated to make a case against all offenders simultaneously. It has severely restricted resources for bringing major cases, and will bring them in the order that makes the most sense to the DoJ.
The government cannot, logistically, make a case against every violator of a given law simultaneously. As big as they are, believe it or not, they are generally strapped for enforcement resources.
So, in a sense, all federal law enforcement is "selective". It of necessity has to be.
The principal then is, the selection criteria should be aligned with fairness and the best interests of the country.
Targeting the most lucrative violator of a given law seems entirely reasonable.
This could turn out to be a very dangerous precedent to make, and not good news at all for any listing related startup.
Even if you don't want to host these illegal ads, and you put measures in place to block and remove them, you might still be held liable. I'd hope they go after the main violators (both of the law and Adwords ToS) and not their unwilling messengers.
As the wiki article notes, this kind of argument tends to not be very successful.
One guidepost on when Equal Protection claims are more likely to be successful is the notion of "strict scrutiny" (http://en.wikipedia.org/wiki/Strict_scrutiny). For instance, if prosecutions are taken against people of one ethnic group, then these prosecutions are subject to a higher degree of judicial scrutiny and are more likely to be overturned.
As the parent comment to this one notes, targeting the most lucrative violator would seem to not be prejudicial in this way.
You are assuming that some people/companies are not touched by the law at all for the same activity, but often it's just a case of not being able to bring all those cases simultaneously.