Early on in the dev process, they said they were making you immune to being PVP attacked while on a flying mount. This prevented you from being polymorphed and falling to your doom.
Flying Form is immune to Polymorph, yes. But its not immune to being attacked.
We haven't actually decided yet if players will not be attackable on a flying mount. I believe we'd rather have people be attackable than not but again, it all comes down to what the results from testing come down to.