non-DOT appr tires, why?

["Richard G. Ballantine" <rgb@frontier.net>]

At an Alfa club gathering in Colorado in September a couple fellows
were admiring the Goodyear Blue Streaks on my car and asked, why, if
those tires bring me safely back to the paddock after lap after lap on
the fast down-and-back track that once was Stapleton airport, they
aren't OK for street use?
    I said I thought it was because of the sidewalls, that the sidewalls
lack....lack....lack.... What should I have said?