[josephcsible]

For characters between U+0020 and U+007E inclusive, there's no good reason at all, and it probably means that they're storing passwords in plaintext instead of hashing them, and that they aren't using parameterized queries to protect against SQL injection.

For characters outside that range, there is a good reason: it's hard to type those characters consistently across different platforms/systems, and they don't want you to lock yourself out over that.