My guess is because of the cat in the head pipes. I have a 2011 RKC that I put Fat SHotz on. WIth the cat, the sound is muffled significantly before it gets to the muffler, ergo a straight thru baffle can be used without violating the almighty EPA sound regs (you can see straight through the Fat Shotz). I bet the pipes would sound MUCH louder without that cat. I called the guys at SuperTrapp to ask them about the kit they sell for the SE Fat Shotz that has more discs along with the closed end cap. The tech there told me not to bother unless I install new head pipes as the cat won't allow it to get much louder, but that with new head pipes the kit would allow the full range of tuning that SuperTrapp is known for.
I'm pretty sure you can fit the mufflers on your bike - and I bet it would sound great!
I hope that all makes sense!