@RandomRandyRandoRay the same thing that stops you from changing a few words around in a Batman comic and uploading it as your own work.
Fan Translations exist in a legal gray area. They are be definition illegal, you can't just giveaway someone else's intellectual property without their express consent. But since most of the people who read a translated work do so because they can't read the language it was originally written in, and therefore were very unlikely to have ever bought a copy of said work anyway, most authors and publishing companies are willing to turn a blind eye as long as translators aren't trying to profit from it (this is one of the things that makes groups asking for donations contraveritial) and don't claim they own the original work. Ultimately free fan translators are just trying to spread knowledge of works they like after all, and that could potentially open up a new market for said work if things go well (see the entire american anime sub/dub market). Heck even if it doesn't maybe a couple foreign readers will buy the work just to have it, which is better than the 0 they'd get from them otherwise (see mangdex's retail links).
However once a publishing company sees that there is a market for a work and starts producing their own officially licensed translation, they is no more gray area. Once an official release is out any fan translation is now undercutting sales instead of potentially generating them. Publishing companies have the right and are more than willing to shut down any site that that helps in this, and just leaving it at that is them being nice.