Oftentimes, the people leaning up against the machine as you're playing them don't really mean it. I always say, "Excuse me please, but I'm playing this machine," and they'll back off in a mix of surprise and guilt. It is quite bizarre to me though when they do it as someone is playing.
My guess is that they think they ARE giving you the space you need, as I think to most people, the idea of nudging never even comes up or they think it's cheating. Also, I'm sure they assume that pinball machines would be like other similarly large items in that they are rigid and fixed, not realizing they actually give a little bit to allow for nudging. When they're right up against the flipper buttons, on the other hand...I have nothing I can think of besides that they are likely people who don't really respect private space to begin with (or, in rare instances, are subtly trying to suggest for you to leave).
To see what you're doing? Heck, at Chaparral Lanes on weekends, they will CLIMB ONTO THE MACHINES' PLAYFIELD GLASS, even when you're playing them! They're too busy chasing each other having too much fun to realize someone might actually be using them! Chaparral Lanes, you see, has a row of five to seven machines, and the kids will usually climb onto one at an end and run across them screaming and squealing. This is rare though; more often, they run underneath the machines or will try to shove you out of the way. (I do remember one case where a little girl's mother saw her shoving me, and she walked into the arcade area, grabbed her by the wrist, and escorted her out telling her not to shove people. The girl was not really listening and was too upset that her mom had ended her fun.)
Between these kinds of incidents, of people leaning on machines when you're playing on them and kids treating them as jungle gyms, it's a pretty dismal sign when the average person just treats pinball machines as blocks up against a wall, bercause someone actually playing on them is such an unexpected occurrence to them. Or maybe it's just me.