Because the law doesn't specify they can't

Individuals from other countries have been volunteering in other countries since time immemorial, dude. Enough American flyers went over to help the RAF they had their own squadron long before the US officially entered WWII. Before 1917, the Lafayette Flying Corps in WWI France. Also, ever heard of the French Foreign Legion? It's in the name. Hemingway, Spanish Civil War, the bell tolls for thee. Lord Byron fought for Greek independence from the Ottoman Empire. And untold numbers of people have joined battles in other countries, whether they were inspired by a cause or simply because they happened to be there.

Get your head out of your ass

soldier is a profession, a job. Working abroad is a thing.

If you think that government can tell people of this particular occupation what to do... Well, good luck telling a heavily armed crowd that they can't do something. That's a nice recipe for a coup de tat you got there. Just another cause for unnecessary infighting. Nobody wants that.

Every dictator would tell you that military is the last thing you should piss off

Does that answer your question?

please learn to comprehend - i did not ask for examples i asked for why they are allowed.

I didn't give you a single example to answer all of your questions. You don't even know enough to form the right questions yet. That's why I told you to pick up a history book.

yes you have lots of heroes and good guys and volunteering when needed is indeed important.

Thats not the takeaway from the example I gave. Its that war is murky. Geopolitics is a constant moving narrative. Its that principle can be more important than civil statute. Its that a nation of immigrants doesn't immediately divorce itself from its prior ethnic affiliations.

i do not trust the US to not pull the rug out from these guys.

That is indeed a possible risk. You're going to be shocked to learn about American citizens that fought for Germany in WWI, and were welcomed back to American with no hard feelings.

so … how is this legal and allowed

"allowed"? Which unit of the US government do you see chasing US citizens into a warzone to stop them?

and what happens when the US turns its back on ukraine?

Again, history book. There is no one answer. History has examples of it going both ways and no consistent answer as to what the future holds.

So again, pick up a history book and look at how prior examples of this play out for not only US citizens, but also other citizens in other nations that we in the USA drew our legal inspiration from.

Along with the reading of history, if you could check your arrogance at the door, that would be appreciated too. Starting from a place Ignorance is no crime as long as you're looking to learn. It is possible to engage in conversations without being an asshole.

Why would it not be allowed, as long as they're not taking up arms against the US? Isn't the US supposed to be a free country?

Why would they not be allowed? Anyone should be able to go defend Ukraine from scum.