I'll put numbers on it to keep it easy for ya...

1) USGT created to provide an intermediate step between too fast (and ugly as sin) TC and (some argue) too slow VTA (which look cool as hell). ORIGINAL class rules did not have a body list...merely a description of what would be a desirable body as-envisioned by the class creators. Those same rules also stated that any offset or style of wheel that would accommodate the spec tire (HPI X-Pattern at that time). Idea was to create a class that was fun to drive and encourage a great diversity of great looking cars on the track.

2) RACERS who wish to win at any cost (as opposed to lower-case racers, like myself, who put the premium on having fun and a "real" aesthetic) showed up and, apparently, started cheating. First it was playing games with inserts, etc., Then bodies started deviating from acceptable standards. The rules still leave the wheels open, but bigger events (at least the ones I've attended) require the Ride pre-mount tires be used (in fact at MHIC they require tires be purchased at the event). Then an "approved body list" showed up.

Easy?