I'm not very familiar with Roblox, but if I understand it, it's an online platform that allows others to build games on top of it, and like Farmville the servers are exclusively provided by Roblox.
In this case, I would say the moral fault lies mainly in Roblox, since they are the ones providing the platform and hence making the decision to exclusively provide the servers (and that moral fault has not yet happened, it would only happen if when they shut down they don't provide an alternative).
If you are someone building on top of Roblox, I don't think you deserve the same amount of fault, but it's certainly something you have to consider. It's also a risk in that building on top of this design leaves you open to being de-platformed (like, for example with the iOS Store if Apple doesn't like your application).
For what it's worth, I don't think Roblox or Zynga are immoral because of this, just that on this particular topic they are morally faulty.
My bar for what should be acceptable as a recourse is also not very high. Providing any form of alternative like LAN play, closed-source server software, specifications or interoperability with other software would be enough for me. The problem I see with the current situation is the "last resort recourse" there is now in most cases, reverse engineering, is legally shaky, generally against the terms of service, terribly complicated and expensive and not guaranteed to succeed if critical parts of the application are server-side.