Window locks are used to prevent opportunist thieves from gaining entry. Any thief who is carrying a ladder around with him isn't going to be thwarted by window locks on upper floors, since he'll almost certainly be carrying something to force the window open with (or to simply break it) anyway.
I've never hear of window locks being fitted above ground floor level (except, perhaps, where there's easy, ladder-free, access from somewhere like a garage roof). They wouldn't really be serving any purpose.
Chris