Both of those are very answerable questions. Exchanges would be forced to delist Hive and HBD unless it was registered - Hive.io would come under pressure to register it both from exchange partners and the state.
Decentralization makes it harder for the state to target Hive, but just being decentralized does not mean there are not targets. The people involved with hive.io are known individuals whom the state can target with legal action. The witnesses by and large are known individuals, not anonymous, whom the state can target with legal action. The developers who contribute on GitLab are mostly known individuals who can be targeted with legal action.
It's a widespread story and mythos among the crypto space that decentralization makes us immune from the state. That is not a very strongly tested idea. In fact, the few times it has been tested, the state has so far come out on top.
Decentralization gives us tools that will make the fight with the state easier, when that day comes. But it would be foolhardy to think that the state could not effectively crack down on Hive today, if it were on their radar.