The FBI enforce local laws, ie US federal law. Cooperation with them sound exactly like compliance with local laws. We might want everyone to "fight the police" in the name of freedom, but in the real world most people cooperate with reasonable police requests, deferring to thier understanding of the law rather than hashing every little thing out in courts. When a cop on the street asks for help, most people do actually lend a hand.
SCOTUS has already ruled that for first amendment purposes, if the government hires a contractor to censor, it is still censoring and violating the first amendment.
The FBI does not enforce supreme laws to which they are subject to, they have throughout their pathetic existence continue to routinely break laws largely with impunity.
Ya. The government cannot pay twitter to do what the government itself could not do. That says nothing about twitter voluntarily doing something to appease authorities. The police normally cannot force people to cooperate with thier investigations, yet millions of people still do. If everyone exercised thier rights to non-cooperation in each and every cicumstance they could, criminal justice would grind to a halt.