1) I don't think anyone is "forced" to participate. The staging and script encourage audience participation, but no one is forced. That seems a bit strong. There are no consequences to not participating. The night I was there, several people sitting around me just sat quietly during those moments. If you're not comfortable participating, don't.
2) I think the audience's waning enthusiasm is the point of doing it as many times as they do. We are the TV audience - who also becomes fatigued as the network turns what was once an expression of spontaneous, pent-up rage into a gimmick to get ratings. Beale's ratings decline as does our enthusiasm to shout the play's signature tag line.