You have to pay if you use a closed standard controlled by a monopoly.. If MS had gone LDAP we could all be playing nicely together, but no, time for yet another standard that they have exclusive control over.
Yes, but authentication is only a fraction of what AD does. For example, the particular aspect that the OP was interested in was Group Policies. Samba doesn't let you do that. Samba is merely an open implementation of the SMB protocol. AD is a fairly complicated directory service which controls many other aspects of a Windows network.
Samba may be able to use AD, but it in no way provides AD services.