selective prosecution may not be illegal, but targeting someone for selective prosecution based on their political beliefs sure as hell is.
The supreme court just ruled 9-0 for the NRA for nearly exactly that.
The SCOTUS ruling you cite was not about selective prosecution, it was about government agencies pressuring private businesses to stop doing business with groups the government didn't like. In that case, the superintendent of the New York State Department of Financial Services was pressuring insurance companies in the state to stop doing business with the NRA and threatened enforcement actions against those companies if they failed to comply.
-PJ