I would argue that if you're taking >25% max HP from conventional sources, you're probably in a LOT more danger than if you're taking that damage from torment. Neither situation is good, but torment is unable to kill you directly, and it's also fairly likely the tormenting monster is near the edge of your LOS, meaning you can probably escape.
However, this exception for Jiyva followers appears to be
quite old, and furthermore looks like it dates from a time when TRJ actually
didn't expel jellies from torment. A later
commit indicates that the developers themselves weren't quite sure what the intended behavior was here, but they opted to make torment never generate jellies.
Basically, this looks like a case of "it's (sort of) been this way for around a decade, and no one quite knows why". But as I say, there is a potential gameplay justification for it (torment is generally less dangerous than "standard" attacks doing >25% max HP damage).