It's the IRS's job to collect tax revenue. It's not the IRS's job to protect its own existence. That's politics, and the IRS should not be involved in politics.
I think you are misunderstanding. I believe the parent's point is not that the IRS is targeting anti-tax groups out of umbrage or fear, but that people who loudly declare taxation to be illegitimate are signaling, "Hey, I'm especially likely to try and evade paying taxes."
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.
You're using an extreme example, and I think a lot of people will agree in that case because it's extreme, but might have reservations about applying the same logic elsewhere. For example, should it be ok to single out people who advocate marijuana legalization for drug crime investigations?
There has to be some restraint, or else people will be afraid to participate in the political process.
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.
This wasn't a criminal investigation and is not comparable to a criminal investigation.
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).
And if there were a group dedicated to drug legalization, we'd be fine with the DEA targeting them for arbitrary surveillance and investigation. Right? Wait, we wouldn't? Hold on, something is not adding up.
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.
Isn't that the same argument that people use for race discrimination regarding the use of police focus? They reason in a very similar way that its better go after groups like black poor people, than say white middle class.
Why spend equal amount of time investigating everyone? Because the alternative is worse.
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?