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.
Lots of things on long cooldown can be interrupted (death from above for instance), and big deal even if master/strike ravage wasnt interrupted it's still going to be on cooldown. And tell me what else is their to interrupt when a warrior is attacking you? Can't interrupt choke if you're on the receiving end. Having to fill a warrior's resolve (knockback, stun, whirlwind) to get out of a big hitter like that is bad.
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.
Meh, hardly, it's the bread and butter attack of madness, we have no "basic weapon attack" to fill in between stuff. A madness sorc doesn't want to use lightning strike except as a last resort, and if wrath proc is not up, then you'd have to channel that too, which yet again is vulnerable to interrupt.