This is a bit odd if they are cooking meat and Rebel on the same grill. If one assumes that a store has one grill (not option to use the same one), it would still be possible to put up a divider on the grill to have a meat and non-meat cooking area. Based on the number of non-meat burgers sold, the area for each product could be such that it meet the expected demands.
In addition to this, I expect that the same preparation surfaces, cooking utensils, washing up water etc are used for both products…with the potential for cross contamination.
Maybe the issue is not the tools of trade, but the kitchen-hands being diligent in separating non-meat and meat areas/tools.
I also wonder how much cross contamination occurs from using the same grill for both. I also expect that many hard core vegetarians/vegans are overly concerned when visiting their meat eating friends for a BBQ with sharing the same BBQ/grill. We have some friends who are hard core vegetarians and they are not overly concerned about sharing the same BBQ when meat is cooked or in the past has been used for cooking meat…but do get concerned about being served a meat meal.
I also wonder if there is a real issue with Burger King, or some extremists making a point about the burger chain as a whole…namely naming and shaming for serving animal products.