Indeed, they will only have one IPO. But the PR backlash can seriously affect it, if it's made clear to everyone that the execs are crooks, and that they'll never be able to hire talents again.
Moreover, Zynga execs' move was likely intended to increase the total worth of their shares; if they manage to claw 20% more shares, but destroy 30% of each share's value, they've hurt themselves even in the short term (figures made up for illustration, obviously).
Actually, Groupon's execs are widely perceived as crooks.
Their sales teams are apparently very aggressive but that doesn't make them crooks.