Hi WU,
The 'Change Frontpage Contributions' ACL setting actually determines if the users belonging to that usergroup is able to post their blog post to the frontpage.
If that ACL and this setting(
http://take.ms/HxwIN) is enabled, the author can see this(
http://take.ms/iKRPg) in the composer.
If that ACL is disabled, the author will not be able to see the setting and the frontpage setting will be set to true by default.