All their media is hosted under the redditstatic domain, and as far as I can tell, that's hosted on AWS. (There's actually also a redditmedia domain, which they may also use, but that's also on AWS).
That probably means that Reddit can get away with saying they don't host any of it. They merely point their web addresses at the third party host.
It's not just about primes, it's about proving the technologies and techniques needed to verify such a number is prime, which might then be extrapolated to things unrelated to proving things prime.
For example, GIMPS (the organisation behind this find) was a great example of distributed computing long before people had multiprocessor supercomputers in their homes.
But let's not forget the hobby factor. You don't get to decide what other people do for fun. If they want to lend a portion of their computer's runtime to a distributed computing project, that's up to them.
Some people climb tall mountains, and that's not of much use to anyone either.