I don't even know why cruising ON A RACE TRACK is even a thing.
Because there is a LOT more to do in GT besides straight up circuit racing.
Drifting, Touge, Cruising, all frequently drive in the reverse direction for a variety of reasons, but mostly just to add variety
I don't understand this - if someone is drifting the track in reverse don't you just set the track for reverse direction? I've only seen this icon if I'm going the wrong way round the track, but if I set the track to reverse direction I'm now going the right way and don't see the icon.
GT5 used to be plagued with kids driving the wrong way round tracks while people were trying to race, hopefully this doesn't encourage that!
I know Sparco answered you already, but I'll also add that only the GT original tracks can be done in reverse config. Any of the real world tracks, along with Cape Ring, Matterhorne, and maybe one or two more, do not have a reverse config.
As Sparco said, we drift the wrong way often when doing sections at places like Suzuka or GVS (bear in mind, that both the old Suzuka D1 line, and the Fuji D1 line consist of running their respective sections of track in reverse. Since we have official D1 cars in the game, and since GT is a sponsor of D1, I don't think this should be too much if a stretch for PD.)
Also, there are some tracks which are simply better to drift in reverse. The best example I can thing of is Silverstone's Stowe Circuit. It just flows way better in reverse, and provides the opportunity for two massive entries, which you don't get when running forward.