I think you’re jumping to some conclusions that aren’t necessarily merited. We don’t know what DHS did or didn’t do beyond this letter, so I’m not going to speculate on that.

It is completely legal and common for LE to ask someone to stop doing something that approaches the limits of the law or could potentially lead to trouble. We have had recent attacks on federal buildings, in that context, asking someone not to incite further violence is hardly an abuse of power. I’m all for law enforcement reform, but this is a stretch.