Good post, although a bit sensationalist.
But BetterBets is limiting the User to a chose a number between 0 and 2,147,483,648.
Assuming that is true (I didn't check) you are correct, it would make betterbets not provably fair. This is almost certainly an oversight on their part, as for them to cheat would require a collaboration between them and moneypot. If they use the same system for rubies (I didn't check) then cheating is a lot more possible.
Actually if what you're saying is true, BB needs to fix this ASAP because it means a) they're not provably fair, b) It would allow MP to cheat their customers.
I strongly, strongly doubt BB has used this to cheat. But you would be correct, they are not provably fair if you can only pick a seed of half the range.
I wrote this a while ago:
https://gist.github.com/RHavar/a6511dea4d4c41aeb1eb which is what MP casinos really should be using (client side) for picking client seeds. I'm not sure why anyone would use anything different.