Does the user you use has the RMAN privileges to backup and restore. Check that on. One other thing is that you might have explicitly assigned the roles that allows the user to create the table spcae, which I beleive would allow him to create datafiles. Folks correct me if I am wrong, i think to recover you have to be a sys, which would have all the database privileges by default. Or the user have to be a DBA.
I hope this would help you in some way,