In the first place, the players shouldn't be encouraging this by giving their shirts to pitch invaders, who shouldn't be on the pitch.
I saw the incident and it seemed to me like the fan was resisting as they were trying to get the shirt off him, which they eventually did and it was given to a kid in the crowd. A few more idiots ran on the pitch afterwards (not all City fans as there appeared to be at least one Southampton idiot as well) and a smoke bomb was thrown/at stewards on the pitch.
One of the things I hate is the criminalisation of football fans. The police assume we're all drunken, coked-up wannabe hooligans to start with, which doesn't help matters. It's ridiculous that I can sit in my seat at the Etihad at any other event and have a drink, but I'm a criminal if I do that at a football match. There's been numerous other incidents of heavy-handed and even illegal treatment of football fans by police and stewards over the years and I'm not going to defend that in any way.
But idiots running onto pitches when they know it's illegal just reinforce the negative view that authorities have of football fans and a few idiots doing this could lead to us being fenced in again. So while excessive force to restrain someone or while they're restrained is completely unacceptable, it would be much easier if people didn't put themselves in that position in the first place.