they can be, but most singlespeed bikes use caliper brakes (like the ones on a geared road or mountain bike).
then there are fixed-gear bikes (there seem to be more fixie riders than ss riders in this forum).
fixed gear bikes have no freewheel, so there's no coasting (just like a track bike), which also means that you can ride without brakes. you stop by slowing down the pedals with your legs. it's fun.