[_dain_]

I suppose one legitimate reason might be to safeguard you in a scenario where you have to enter the password on a nonstandard keyboard? Also, if they can be easily confused with some other characters it might make sense to disallow them just to remove a headache for support staff trying to deal with people entering the password wrong (e.g. en-dash vs em-dash vs hyphen).

But they're probably just storing it in plaintext on some legacy system that can't handle certain characters. Or the plaintext goes through one of those systems on its way to being hashed and salted.