I think it is because it is not implemented the way (I imagine) URL shorteners to be implemented with a database back-end to map the short tokens to the actual

I think it is because it is not implemented the way (I imagine) URL shorteners to be implemented with a database back-end to map the short tokens to the actual sites. Instead it just performs a reversible transform on the URL. This way the site is really just a static website (and I imagine a lot cheaper for the owner who can't be interested in laying out a lot of money for this). But this is just a lot of speculation on my part which means I'm probably wrong about at least one aspect.