Whether or not it's damaging the machine is moot- our events use IFPA rules without an exception for bangbacks/death saves, so if it's a bangback it's illegal. Take it up with those guys if you think it should be allowed and maybe we can watch some 3-hour games at Pinburgh next year.
My take is that it if you showed it to most players, they would say it was a bangback. The minor distinction that the ball isn't on the apron doesn't hold water.
As for damage to the machine, I don't think bangbacks are particularly bad for the games, although they aren't great for your wrists if you're not careful. Allowing players to pound on games tends to escalate very quickly though and should be discouraged.