I have done this too. I sent 1 bch to a btc address on localbitcoins. The problem is they can't get your bch back because it would compromise the security of their system to extract a wallet privatekey just for you.
This is what I was told. I guess it does make sense.
it doesn't really compromise any security if it is done correctly and by the right person. a lot of cases on different exchanges have been solved this way in the past. the problem is that it is a lot of work to recover these coins so exchanges rarely bother with it unless the amount is significant enough to justify the effort.
It's not dangerous if done correctly, but despite having done correctly thing, still not rule out possibility for hackers to infiltrate their system. So, when they extract one of private key, they are a little stretching their security to access private key, well we will never know that hackers took opportunity at that time. If value isn't too much, I think it's better for user to just let it go, but if it's worth a lot, the platform should help restore it.