This behavior is by design. In addition to adding the user account that is running the setup as a local administrator as documented in How to: Install SQL Server 2008 (Setup), the setup user account requires the following default user rights for the setup to complete successfully.
Local Policy Object Display Name (User Right)
Backup files and directories (SeBackupPrivilege)
Debug Programs (SeDebugPrivilege)
Manage auditing and security log (SeSecurityPrivilege)