Technically, the Group Finder tool is intended to help folks find groups for quests, dungeons, raids, etc. Not really for guild recruitment, as that has its own tool.
That said, there isn’t anything specifically prohibiting them from advertising anything other than for quests, dungeons, raids, etc… as long as they aren’t advertising runs for sale. Keep in mind though, if such use becomes an issue, we often will change the functionality or policies around it to better serve the community and intended functionality of a system.