Wow, I didn't know that. It would be super funny if someone could exploit that haha. But anyway, I think it's more important that units don't stand in light during night missions, than it is for them to stand in the dark during day missions.
Edit: I tried a mission site with globalshade=9, but could not find any shade=10 tiles even inside houses, so I dunno if it's possible. In any case, I'd still prefer if the script isn't ran in day missions.