I think it depends on what kind of "singling out" is being considered. What the IRS agents did in this case definitely crossed the line. But they
should be allowed to scrutinize applications from suspect groups, and probably also be allowed to ask for more information to the extent that they would in a random audit. Further than that should require strong probable cause and warrants and all the other due process that was ignored by the IRS agents in this case.
A drug case would probably get to the point of needing a warrant much sooner, since a pro-legalization activist isn't usually directly initiating contact with the government, so the government has no business asking for information without a warrant. But if a pro-legalization activist makes public statements about their illegal activities or intentions, it's okay for the DEA to keep an eye on those public statements and to use them to try to get a warrant.