He was able to provide a justification, however thin, which he presumably can't in the case of St Louis. Not that I disagree with the general sentiment. He's only doing this as a political stunt and St Louis wouldn't serve that purpose as well even if he could somehow swing it legally.
I'm sure there are also federal buildings in St Louis; the justification from California works almost anywhere.
But critically, the trial in which the legality of that action is considered is happening the week. Whether or not the action is judged to have been a constitutional violation ultimately doesn't matter; the administration did it, and even if the court rules against the administration, it will have been two months too slate. Effectively, the president has demonstrated he can federalize the national guard whether or not the governor consents for long enough to score whatever political/media points he's currently fixated on, and if the legal system stops him, he will have moved on to other issues.
