Why would the IRS spend an equal amount of time investigating everyone, when certain groups go out of their way to paint big targets on their backs?
Similarly, if there were a group dedicated to the idea that seducing little boys was OK, you could expect it to get some extra scrutiny from the FBI. This is not because the FBI is made up of little boys who feel threatened, but because these people are waving a big red flag that they're probably engaged in activities that the FBI has an interest in.
There has to be some restraint, or else people will be afraid to participate in the political process.
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.
I wouldn't be concerned that a group whose politicians have on several occasions advocated: armed marches, intimidation of the other party around polls, refusing to pay tax, shoot census takers, or preparation for civil war - was "afraid to participate in the political process." I would rather be concerned that it was causing others to be afraid to participate in the political process. This IRS gaffe is covering a history of anti-democratic conduct from the very group beating its breast (even as its groups enjoy the nonprofit status which the IRS gave them long ago).
Basically, the two things are not very much alike, because the DEA isn't supposed to be able to initiate a full-scale investigation without a warrant, while the IRS has a bit more discretion in whom it scrutinizes.
Why spend equal amount of time investigating everyone? Because the alternative is worse.