My neighbor makes the Spanish Fly sparkies and they are definitely sweet. He told me the reason that they aren't USFS approved is because you have to approve every single bike make and displacement for use so it's a huge PITA and costly. Also, most others probably aren't officially approved either...
That said I've run them for years on my bikes and have never had an issue. FS rangers have checked them before as well.
I also had them at the UTE cup the last few years with no hassle and saw plenty of other RMTA folk with them.