A downvote isn’t “I don’t think this should be here”. It’s just “I don’t like this”, don’t agree, thing it’s bad/wrong, whatever. Up/downvote systems work under the process of the wisdom of the crowd, and as a result, works under the assumption that while the ways in which people use these tools and the reasons may be different, having a simply up/downvote system means they’ll average out into something more useful. It doesn’t work all the time of course, but it does work generally.
Ya, never trust US companies. Their government’s crazy to jump in and take anything they want; you may not even know they took it.