Because the carbs are large.. I think that is EXACTLY the point I made in my first post.
Large throttle plates open quickly and rush FAR more air than could be matched with fuel, so they squirt a small bit to give all that air something to burn with. A small moped carb, even when opened quickly does not have the volume or velocity to surpass the jet's ability to mix. Even my 24mm flat slide has only been able to do it ONCE, when I whacked the throttle as fast as I could at a stop and it coughed and almost died. Never once while riding has it been an issue.
Got any more comparisons to try and make?