If Pizza Hut employees are involved in shootouts, the company will get sued out of existence by the soulless lawyers.
You should have heard the lament from the hyphenated boy's mama in the Indianapolis Star, complaining that the shooter should have been charged with SOMETHING for ventilating her son so well.
Bingo. No question Pizza Hut's rule in this regard is prompted by liability concerns. In addition, I wouldn't be surprised if the rule is also required by Pizza Hut's Insurance carrier.