Not sure why, I've never heard that... I ran double adjustable Koni's or Strange Eng. shocks & Lamb Struts, for years on the street, also my QA1's on my last 68 RR, I drove to & from the track 12+ miles each way, with the track set up settings, from Rancho Murieta to Sacramento raceway {long trips it was trailered, it also bounces in the trailer also, working the shocks too}, with some cruising occasionally too, no issues... maybe it's because they don't want to warrantee them or something... not really sure why... maybe a wives tale... in the Strange Engineering Overview on Jegs it says double adjustable shocks for racing, street, Pro-Mod... the QA1 Overview says, street-rod, street cars, racecars, autocross, road-race etc. too... maybe we have a "real shock expert" here that can enlighten us/you... It's news to me, but that's nothing new...LOL...
I'd say wear & tare more than likely...