To answer your question, it’s because PR’s one of our territories. Just like Guam or American Samoa.
To remark on your comment, why the heck would we do that? I don’t see any reason I would vote to allow Puerto Rico independence. And why in the world would anyone want to pay reparations? We’ve improved their infrastructure, and they certainly don’t owe the U.S. “proper” anything.
Note: By U.S. proper, I mean the actual 50 states comprising the union. The way I wrote that seems kind of confusing.