I'm not really clear what "supposed to" or "should" mean here. I'm pretty certain the audience would've understood it was expected that Abramović would sustain some sort of injury or discomfort, in line with her other pieces. I very much doubt anybody would subject themselves to a six hour performance without any idea what to expect. If nobody had obliged, I think it's fair to assume Abramović would've left disappointed, and would probably not be the conceptual art superstar she is today.

There's a whole world of consensual injury that to be honest I'm fairly personally prudish about, but nobody is making either of us participate in it. If people want to do it, if I'm honest I'm slightly judgmental but I feel like I should be less so.