Since tooth is singular for teeth then why is a toothbrush not called a teeth brush? Unless a person was down to a single tooth in his or her headball then they would brush their teeth with a teethbrush.
I Guess the Funda the Toothbrush companies follow in with the name is that their product aims at cleaning each tooth and as a whole teeth's.So they named their product as Toothbrush.
Note:If not enough then only the tooth brush company can help you
The wonderful flexibility of English permits a noun to modify a noun--something that isn't allowed in many other languages. In America we almost always use a singular noun, even when logic suggests a plural (Brake Service, Fire Department, Cigarette Machine, Dead Letter Office, etc.). In England they sometimes use a plural (e.g., Complaints Department).
_________________ Mr. Fussbudget
True wit is nature to advantage dressed,
What oft was thought, but ne'er so well expressed.