I’m a DC resident and your post is leaving out a lot. Yes, federal agencies have police to protect areas but they are in uniform and have a specific mission. Sometimes those intersect - a couple of years back so joyrider ended up getting chased by something like a dozen jurisdictions due to their route - but in general you aren’t going to see, say, FBI agents hassling people at a random park in DC which is technically a tiny federal (NPS) property, and their ID is not concealed.
What has people concerned now are the new personnel who normally are not here, hide their identity, and are claiming to provide assistance which the city has not requested. If this was done above board, it was far less controversial.
As a fellow resident, I disagree - I've actually never been pulled over by an MPD officer, but have been pulled over a few times by federal officers for minor driving violations (was pulled over and ticketed in DC by the secret service, far away from the downtown area, over having forgotten to turn on my lights)
What has people concerned now are the new personnel who normally are not here, hide their identity, and are claiming to provide assistance which the city has not requested. If this was done above board, it was far less controversial.