I am not a Japanese and I don't live in Japan so it's only information from Wiki. If gambling is not legal in Japan, and if Betpanda wants to provide their services to Japanese, they must have many mirror sites as Japan governmental authorities can work, find and restrict the main and mirror sites of Betpanda if they know.
It would make no difference, and also the site is not a mirror, if it were one, you could log in with your normal betpanda account, which you can't, as I tried it just now!
And speaking of banned gambling in Japan, I find it funny that they actually do have the bets odds for Japan's horse racing, one of the few things for which gambling is not banned in Japan, too so bad I need to wake up at 6 am to bet on them:

