I think this is illustrative of a fundamental difference in philosophy between employment in the USA versus elsewhere, and one which favors economic growth and innovation over social stability.

Requiring cause to fire someone is business-hostile at best. Business can not improve efficiency, pivot, or generally adapt to changing markets if they are not free to hire and fire at will.

The one major point most "liberals" (in the American sense) miss regarding employment security is that employers are far less likely to take risks on hiring - whether in quantity, salary, or experience - when they can not fire at-will. This leads to less employment overall, not more.

The arguments are similar to other well intentioned, but seriously deleterious policies, such as rent control and Calfornia's "Prop 13."

That said, ethics and treating people well are very important. I find Zynga's actions unethical and think there is a good chance they should be held civilly liable.