It's hard to say if he's been using them
without permission, or just until the musicians have discovered that he has and then asked to no longer be associated with his brand/withdrawn the blanket approval that was previously set up.
As an example, at a national 'do' for a sports organisation that I'm involved in, it used to be traditional to use We Are The Champions right at the end of the music-punctuated prize-presentation, the lighting/sound box technicians having it amongst their stock of PRS-paid-for tracks, for event use. I have no idea what paperwork may have been needed to do, post-event, to state "We played this tune, please apportion credit from the licencing fund to Queen's management company" or what happens.
But if Freddie (still alive)/Brian/etc had heard that we had used their music and had
dissaproved of being associated with our sport (unlikely, for several reasons), then the Nuclear Option might have been to deregister from the whole "any legitimate fee-paid venue can use the track at any qualifying event" process, perhaps, while we'd probably have stood down our practics if merely informally asked by a representative (but then we weren't total inconsiderate bastards[1], further decreasing the likelihood that the request would even be
made).
It sounds like Trump's 'rally machine' has been using the standard "Now That's What I Call A Political Rally!" set of CDs, prelicensed songs, dirges, melodies and stings available to be chosen from for all those who are in that category, large or small, likely as a small paragraph somewhere in the forty-third page of a standard music-release contract offered to most artists.
So, what to do?
1) Ask nicely. Seems to have been tried before, from what I know of the Stones's battle. Probably tried this time but already didn't work.
2) Disentangle from the whole thing with the Nuclear Option. Takes time, probably takes money (buy-back, various related punitive cancellation/re-contracting fees, that sort of thing) and all for
one instance in amongst so many less controversial or far less prominent uses.
3) *mumble mumble mumble* Injunction *mumble mumble* Highest court you can afford to petition *mumble mumble* Money for old lawyers...
4) Do any/all/none of the above while being very vocal that you are
not happy with the situation, not complicit, by jo means a supporter. (This part can cost the price of a Tweet, which these days is probably only a few Followers who "used to like you, man, but then it turns out you're bought out by the Deep State, shame on you!"[2], and maybe a few others realise that you're still alive and start to Follow you.)
Whatever is happening, to me, it just shows how few people I like actually like him (always nice to know that your favourite '50s, '60s, '70s, whatever artist/band/supergroup aren't ready to do
actual warm-up for such gigs). To others it reinforces the polar opposite. I'm not sure it changes much, but then I've not been overly worried by the few of 'my kind of person' who have indeed openly allied themselves with #MAGA, at least the ones I remember learning about[4] and - moreover - I'm not the kind of person to suggest anyone who doesn't agree with me and don't like 'reality' should leave my (or, as applicable, their) country when I haven't left any country myself whenever the fortunes were reversed.
[1] We actually phased out the track years ago when it was pointed out (by a Champion) that the line "No time for losers..." was perhaps a little
too jubilant a phrase and not in keeping with our sport, where 'losers' do indeed get a time and can always hope to better it even if they aren't going to be parading round the auditorium with a huge silver-gilt trophy anytime soon.
[2] ...but written less coherently. Probably grossly misspelt[3], and uses the word "cuck" (or should have, but misspell it).
[3] That would be 'misspelled' misspelled over there, but I know that I haven't misspelt 'misspelt'.
[4] Of course, I had to check this out. Scott Baio? Really? And surely not Dean Cain! Well, John Voight already trashed the classic Jim Phelps character for me, I was never into Rosanne Barr, and most of the others listed being irrelevent to me, unknown to me and/or just plain weird like with Malik Obama.