A couple of things: season is starting 7th January in Japan, so preseason is on.
On twitter GeeRob was saying that almost all these contracts have injury clauses: if you get injured playing for some other team, they can tear up your contract.
Combine these two factors, and you get a situation where a player might take a risk if they're far enough away from preseason to recover, but not into November when they're expected to do at least 6 weeks of preseason.
(It would be churlish of the Japan Rugby nuffies to stop players representing at the Tokyo Olympics
)