Michelin Pilot Sport Cup 2 tires are fairly commonplace for performance cars and included from the factory on many models. I wouldn't call them "barely street legal"
PSC2s are optional on a lot of high performance vehicles and in many cases standard with the top performance package.
They are also pretty much as close to a slick tire as you can get without compromising all wet performance. There are tires that are more "barely legal" but there aren't many.
The reason they're so widely offered on performance cars is that they are basically cheater tires. They're street legal and grip like hell, but they also cost a lot, make a lot of noise, wear incredibly quickly, and can't be used in cold temperatures. I'm not aware of a single car sold with them as standard equipment which doesn't also offer a less insane tire as an add-on.
Basically they're used so automakers can get good numbers, no one in their right mind actually daily drives these tires. The PS4S used in the original test is generally regarded as one of the best tires for a daily driven performance car.