For clarification: Dubs are NOT allowed, switch tricks are.
The problem is, you need a very long inrun to have time to pop a 180. Especially at the Nor-Am and World Cup levels. You are traveling with so much speed, and the inrun is usually far too small to do a 180. Also, except for the level of skiing with like 10 year olds. They may have a long inrun. But at the Nor-Am level, there will never be a long enough inrun. When I was 12 though, I did a SW 5 because I had fallen in the middle section so my run didn't matter anyway.