I remember reading about vegetarians mitigating this very concern years ago when Burger King put out its first veggie burger.
In 2002, the company already had a response for this situation. "'The Burger King VeggieBurger is not positioned to be a vegetarian or vegan burger. It was designed to be a meatless alternative.' These were the words VIP heard when we called Burger King headquarters in Miami, Florida."
2005: "The VIP tasters sampled the original BK Veggie Burger that was warmed slightly in the microwave to avoid contamination by the animal products cooked on the grill."
2002: "Vegetarians who order the BK VEGGIE should specify that they want it cooked away from the Whoppers. To make certain, they should request it be microwaved."