I think the intention is to impose the same restrictions on the "login" password field as on the "sign in" password field. This gives a hint to users as to why

I think the intention is to impose the same restrictions on the "login" password field as on the "sign in" password field. This gives a hint to users as to why their entered password is incorrect, and does not slow down an attacker. This is particularly important for systems with unusually restrictive password fields.