I've heard (but not confirmed) that it is expense related. If you wholly enclose the toilet in a room, now that little room needs its own fire extinguisher nozzle (and maybe other regs?) but if it's mostly open, you only have to make the overall room safe.
Not ask European toilets are enclosed to the same degree either. Many walls don't go all the way to the ceiling for example. This should enable sharing a fire extinguisher nozzle. Almost all of them have properly closing doors though.
