If the spot market is manipulated, the futures will reflect the manipulated price. Though the SEC doesn't come out and say it, it looks like this is a concern around custodianship.

Bitcoin futures are hedged at the periphery. If a trader loses their Bitcoin, they are unhedged and go broke through usual channels. Bitcoin ETFs hold that risk at the centre. If they lose their Bitcoins, we have to deal with trading halts and moms-and-pops losing their shirts and the family office that quintuple-leveraged their ETF bet and is threatening to take out a bank.