The contract is actually signed when you install the game. It is also known as an EULA.
TL;DR
You do not own anything in blockland, you are just allowed to play it for $20. Piss off Badspot and he can make it so you do not play again.
Yes, it includes mods, builds, or anything else made "for, in, or from" Blockland. If anything should happen to break the law while using said 3rd party material, you get the blame, Badspot does not have to deal with it. Yet it is still his.