You're a child until you're 20 in Japan and it's been that way since the Meiji period (up until then it was 15ish for boys at least, girls until they got married, I guess?) and while you're a child obviously the laws regarding working as a child will apply? I don't know why they put the "after 18 you can work past 10pm" thing in, but as there are talks about making 18 the coming of age, I would assume that's it? Also, because that's when people have graduated high school, maybe? I don't know.
As for appearing on TV, I doubt there are any shows that are recorded and will have someone leave in the middle of it -
unless the recording time passes 9-10pm because then it goes against the law. Which is why the U18 members weren't performing live at Coming Soon as that airs past midnight. The AKB members might just have been days from their 18th birthday, or something, really.
Or, they didn't care too much. Johnnys have been ignoring laws like that too - allowing under-age smoking and talents working late. So who's to say another successful group of many idols won't. UFA might have done it in the past too when it came to people around 16-17. But with all their scandals in the past, I'd understand if they wanted to be super careful and let Risako & the C-ute girls skip this one. Who knows, had Risako been born a week earlier, she would have been a grade higher in school and maybe then they would have let her on simply because that would have put her a year closer to "adult" in terms of education and whatever.