I would think it's valuable for the entire Bitcoin network to have more merchants accepting it than less, so how is Zynga accepting Bitcoin a bad thing?
They are still only accepting USD: just making it easier for people with bitcoin to give USD to them.
On the other hand, if Zynga were holding BTC, paying employees in BTC, and those employees started consuming with BTC, then they would actually be "using" it. Otherwise it's the same exact thing as punching in your credit card details.
You can pay for something with euros in the US as long as they accept it, even if after all they do is go to the bank and convert to USD.
They are not accepting real USD. They are partnering with a company to handle whatever needs to be done to give them real live USD they can put in their bank accounts.
hint: This is how merchant accounts with payment processors work.