Japanese culture in a nutshell reallyAbout being "forced" to work or not.
I know there have been talents who have implied or even straight-out said that even Fes participation is not mandatory. (I I don't have the source, this is all second hand information.) But I still have some doubts on that.
Sure, maybe on paper it is optional, but there is probably huge "pressure" for a talent to take part in it. Imagine if someone just opted out of Fes, unless you have a very good reason (like Haachama opted out of 5th Fes because she was sick, but still appeared as a guest in the end), I don't think the fanbase will react well.
You "don't" have to drink with your boss at the end of the workday, but you are expected to.
At the end of the day though, we really don't know what's going on, so what ever rrats we think of would actually help make sense of why Shion is leaving.