Like everyone else has said - part of the game. I usually go quite a bit easier on other's pins as a courtesy.
What I CAN'T stand, is an aggressive act when the ball is NOT in play. Banging/slapping the machine on the side, lock bar, or glass is simply unacceptable and childish. It's fine to give the machine a reasonable slap when it's in an attempt to save a ball or improve it's position. It is not alright to do this in some sort of display of what you perceive as the machine's wrongdoing.
I used to golf with a pro who would laugh at people when they threw their clubs - then he'd say "clearly you aren't as good as you think you are!" He always reminded people, "you looked stupid when you threw your club, you look even stupider when you have to walk to pick it up.." The same holds true for pinball, we're all subject to the laws of physics. No sense in going into toddler-mode over it.
By the way, I think we should all start shaming these people.. How about a collective "aww" or "what gives" when people act out on a machine in this way?