The best part of the fediverse is that anyone can run their own server. The downside of this is that anyone can easily create hordes of fake accounts, as I will now demonstrate.
Fighting fake accounts is hard and most implementations do not currently have an effective way of filtering out fake accounts. I’m sure that the developers will step in if this becomes a bigger problem. Until then, remember that votes are just a number.
This would actually be a bit more difficult. So first it would be easy for me to set up lemmy1.derproid.com, lemmy2.derproid.com, etc. but if you could just defed from *.derproid.com it’s no problem. However setting up lemmy1.com, lemmy2.com, etc. is more expensive because you would need to register and pay for each of those domains individually.
That’s not to say it’s impossible but there is a bigger barrier to it.