I took the line stating that "Only Fleet Command knows why" there's a death penalty for visiting Talos IV to mean that there was something else that happened that we aren't made aware of.
Perhaps the Talosians made another, more serious attempt to capture humans, which Starfleet judged to be a clear and present danger to the Federation.
Their power was
staggering. They could reach out to a starbase light-years away and project a perfect illusion there.