Well, I'll tell you why. The Burger King commercial she did in 1982 was the first commercial to ever mention a competitor by name. Consequently, McDonald's sued her as well as Burger King. She also couldn't enter a McDonald's unless she was in disguise due to truth in advertising ("I only eat at Burger King" was one of her lines in the commercial). Ironically, McDonald's was a sponsor of the WB's hit TV show "Buffy the Vampire Slayer" (1997).
And while we're with the trivia, I just found out this bit of info: SMG was invited to reprise the role of "Buffy Summers" in Buffy the Vampire Slayer (2002) (VG), and also in Buffy the Vampire Slayer: Chaos Bleeds (2003) (VG). Turned down both jobs because she had insisted on seeing their scripts, and never did.
Finally, she broke a bone in her hand while filming the "Buffy the Vampire Slayer" (1997) episode "Homecoming".
Ok, done with the Smidge Trivia for now. Until next time :)