Insurrection established that isolytic weapons were banned by the Second Khitomer Accords. Vadic presumably has them because she's not in Klingon or Federation space. If the Crushers stay in Fed space, they'll be nabbed by Fed conspirators. If they leave it, they run the risk of being blown to bits by stockpiled isolytic weapons.
If Starfleet can't do anything about supervillains hoarding banned weapons outside Fed space, you think they'd at least have SOME kind of defense against them, especially if they're an exploratory vessel that would presumably venture outside Fed space.
If Starfleet can't do anything about supervillains hoarding banned weapons outside Fed space, you think they'd at least have SOME kind of defense against them, especially if they're an exploratory vessel that would presumably venture outside Fed space.