it's normal...i haven't figuared it out yet i have asked lots of people why this happens its like a reverse sensor light...happens to me all the timne...and most people i know that have walked around at nite.....i think it was rigged up somehow for people up to no good
yer gonna do somethin bad or illegal you walk up to a streetlamp and it will go out so no one can see what your doing.....
but try watching CONSTANTINE and go out for a nite walk or to walk your dog if you have it and have the lamp go out on you!! your butt will be back in the house within minutes