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.