I suspect foreign agents try to bribe people in submarine crews to sabotage their sub in the event of a strike. If someone onboard could stop the American second strike missiles being fired they'd give Russia a huge advantage. I imagine the Navy have considerer this.
Au contraire! An important aspect of them as land based is the "nuclear sponge" concept: every single nuke spent on a silo is a nuke not aimed at a population center or vital infrastructure. They're out in the middle of nowhere for a reason (well, multiple reasons, but that's one of them). Even if they're all lost they've constrained the opponent's tactical choices merely by their existence, pushing towards the logic of a counter force strike instead of counter value.
Any nuclear exchange would be very bad, but unlike Europe the US does indeed have a lot of near completely uninhabited non-arable land for strategic depth. Makes sense to use it.
Further, they are trapped in a small metal tube with 100 other people in similar condition, and who will probably be....not pleased if any sabotage is discovered.
I suspect it would be very hard to successfully bribe said crew members. (And have them actually follow through with their end of the deal, rather than just taking the money and...not doing it if the war ever happened)
Surely hitting every ICBM silo, B-2 airfield and nuclear submarine virtually simultaneously is nontrivial.