It doesn't have a basis in law. Federal agents can't just turn up, unidentified, and take people off the street without probable cause. There are plenty of videos circulating online (I'd link them but they are not GTP friendly) that shows an unmarked vehicle driving up, agents getting out, grabbing someone, and putting them in the vehicle, before speeding off. That's not legal. If you're being arrested, you have rights and you can't just be arrested for the hell of it, especially by federal agents if you're not breaking federal law.
What's going on is enough to have the US Attorney's office for Oregon call for an investigation:
US Attorney For Oregon Calls For Investigation Into Portland Protester Arrests
This signifies, at best, that there is at least some evidence of wrongdoing on the part of the Federal Government. It's also worth mentioning that Billy J. Williams, the US Attorney for Oregon, was reappointed by Trump.
The Attorney General for Oregon, Ellen Rosenblum, is also
filing a lawsuit against the Federal Government too. If there weren't grounds for a lawsuit, they wouldn't pursue it.
You have way too much faith that the Federal Government is acting in the best interest of its people here. It's not, it's throwing aside a pretty good justice system in favour of doing whatever the hell it wants.