Yes, it's an exploit and yes it's a bannable offense if you can provide evidence that it was used to garner an unfair advantage.
Yes, talking about it on the official forums (this thread) is grounds for thread deletion and disciplinary action.
That said, I prefer to speak my mind. I like this little mechanic and most people only do it for giggles and quite rarely. That said, 170 kph Direwolves are uncool. If you are caught doing this regularly, you should and will be punished for it.
Riding another mech does not make you unshootable. Quite the contrary, actually. It elevates you above cover and more enemies are likely to shoot you - it makes you more vulnerable. In my testing I've determined that it doesn't convolute hitboxes nor does it have any negative impact on hitreg. What it does do however is cause warping/teleporting when the mechs are travelling beyond a certain speed (though this is a rather slow speed, somewhere between 40 and 70kph and seems to depend on the chassis, and it DOES cause obvious hitreg issues)
Personally, I hope it's never removed from the game completely. I'd rather see it expanded on as a proper game mechanic (i.e., a Direwolf trying to ride a Locust should crush the Locust or at least cause a knockdown and severe damage) and implemented alongside proper Death From Above mechanics.
I'm expecting this thread to be closed soon.