Well Sins get a passive 25% shock buff (like why? the can use their sabre to hurt people, we can't), but not only does that tank FL do more damage with 3 stack harnessed darkness, but it heals them too and is uninterupptable! But yes, why the heck madness sorcs don't get anything like this (FL self-healing when death mark on the subject), and I can't for the life of me work out how they can make master strike/ravage uninterruptable but madness sorcs FL isn't. It is as you say, salt in an already very deep wound.
Well that's easy: Force Lightning is a ranged attack with zero cooldown in case of Madness, Ravage/Master Strike is a melee attack with a 27 sec cooldown (assuming everybody takes the Ravager skill). If someone interrupts Force Lightning, you can use it again in 4 seconds; if someone interrupts Master Strike, you can't use it for half a minute.
Having a talent that makes Force Lightning uninterruptable for a short time every minute or so would be okay; but just making it totally uninterruptable is way too much.