These are all summons with forehead protector that I can find, it's quite rare among the summons. For example, Sannin's summons, Inuzuka Clan's Ninkens and the Ninnekos don't have forehead protectors.
Are the headbands are overall just for decoration purpose for the summons?
Weren't forehead protectors are worn by most Shinobi since the summons are technically mini ninjas?
Kakashi's and Guy's summons:
Hiruzen's (Third hokage) summons: