ST classes are only a few years old- before, you went right from Stock to SP. ST was created to offer a sort of intermediate level- a way to offer a class for those people with typical bolt-on mods. no, it's not a direct path. but it's not meant to be...there's not really a direct path from any class to another.
Stock has always allowed any DOT-approved tire- that's not going to change on a national level. Locally, regions may offer any class that they want.
ST, by definition, is a street tire class. it allows all Stock allowances except one- the tires. hence the purpose of the class. ST is the only class that doesn't allow DOT r-comps.
SP is, in large part, ST with r-compounds.
the rules are created with national-level competitors in mind. local regions are free to offer any class they would like, and many do (STX2 in philly, for example). you're free to run 15" r-compounds, but as you saw, it might bump you into a class you don't want to be in. if you want to be competitive, plan your mods accordingly. if not, don't worry about it and just have fun.
