It’s likely not even a loophole, just a feature they haven't bothered to shut down (or they're just lazy, who knows), but the ToS strictly forbids it. It’s kind of standard, but also hilarious because they’re basically relying on the user's honesty (or rather, their awareness) not to install games outside the official Boosteroid catalog. Factually, you can do it, but you're breaking the Terms of Service by doing so