Given that they're on a Starfleet vessel it's entirely possible they signed away those rights.
Yeah, they probably had to sign something. Of course, I'm sure everybody would agree with this one (once explained to them).
"So...they want to kill us all..."
"Can we fight them?"
"Sure...we'd die...but sure..."
"Ummmm...so, what's plan B?"
"We just need to have our own doctors remove our memories of the aliens."
"Okay...that sounds a lot better than death...Count me in..."
I really can't see anybody saying no. Plus, it's not as if Picard hasn't earned a bit of trust by this point... It's half-way through season four.