Comment by nospice

If you want a real answer, it's because the president is the supreme commander ("commander-in-chief") of the US armed forces. He can order them to, and it's unlikely that they would refuse to carry out an operation like that.

If you're asking "why is it legal", that's a somewhat separate question, but the short answer is that the Congress has long abdicated this responsibility and has not sought to reassert it. Basically, they're OK with it, and there's no one else in power in the US who will be upset about the US successfully arresting Maduro.