People with COVID 19 shouldn't be travelling with other people or infecting border guards. So this may be sensible.
Not sure why infected Americans would wish to travel. But as with masks, it's about duties to others, not personal rights.
By the way, I think vacationing is already not open to Americans. So not sure how people travel out of the US in the first place. And if you are an American living overseas, then I think you stay where you are.