Precisely why "trust" should never be a part of an equity negotiation. When millions are on the table people will do whatever they can get away with. Zynga doesn't want another "google chef", but they took full advantage of the "google chef" when promising equity to early employees.
No, but they probably will be promising employees something - and now, everyone knows that unless that promise is in writing, in an iron-clad contract peered over by Hell's best lawyers, it's worthless at best.
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.