Paintballers do a LOT of things that would make a firearm owner cringe.
Shoot at each other deliberately, for one, shoot out the back door for another, many markers don't even have a mechanical safety, we'll swing 'em around in the store not really caring who we're pointing it at, you name it.
Yes, flipping a real-steel rifle at the range will in many cases get you escorted off in a big hurry. Similarly few competitions will let a competitor use a horizontal shoulder holster- in both cases because it inherently allows the muzzle of a loaded gun to sweep the spectators.
But, this is a paintball gun. As long as the user and nearby players are wearing their goggles- as they should be when playing with a loaded paintball gun - the worst they can do is give themselves a bruise.
And technically, the "flipping" technique is pretty foolproof- it would be difficult indeed to close the action and have it accidentally fire while inverted and pointing back at oneself. Not impossible, because I've seen some people do some mighty strange things but it would sure take some weird and probably intentional extra movement.