Trump is threatening to “send in the Feds” because he doesn’t like the alleged crime rate numbers. He believes that he alone can stop crime, and it looks like he wants to do it with martial law and jackbooted thugs.
When Obama “sent in the Feds”, it was in the form of DOJ investigations into police departments who were allegedly violating people’s civil rights, which is a federal matter.
If Trump believes that police departments are violating federal law by stomping on people’s civil rights, and he orders a DOJ investigation to check it out, I’m good with that as long as he has reasonable cause to do so. But that’s a far cry from sending in federal troops to implement martial law, which is what his tweet is implying.