Far from being an expert on the Japanese language...I took a beginners Japnese class some time back and the teacher was a native Japanese lady. She said that "Sensei" referred to a master of a given trade. Though generically it can be used as "teacher" it really refers to someone who is a master at their craft. Using that definition I find it hard to believe that so many can be a sensei.
In every day usage, anybody teaching anything is called "sensei", whether it be pottery or day care - or hip hop street breaking, whatever. Doctors and lawyers are also called "sensei".
Basically, it just means "teacher" - so it's used for anybody teaching because Japanese are anal about using titles to refer to people.
Depending upon where you are and the culture you live in, that may not be the case.