What on earth? Why would that matter? Does somebody lose their vegan powers if a particle of fat gets into their burger?
Restaurants tend to take the strictest interpretation of a diet because that minimizes angry customers.
It is wrong, it a real sense, to have unreasonable personal rules about a restaurant kitchen.
It's wrong to eat meat.
This is all obvious and well-known. I know that righteous indignation is a drug, and it shows mostly on the internet, thank goodness. But we should all strive to break the addiction. The world will be a better place.
And don't be silly - everybody has had to suffer through the lunch-time episode of somebody at the table grilling the waiter about how the food is cooked and what exactly is in it and don't they know they should do it different? While everybody else wishes they'd get on with it because we're hungry. Takes no imagination at all.
At least for me, the point is that I'll try to make choices that reduce suffering. You can't reduce it to zero, but most animal products can be replaced with a similar alternative with a similar price and less suffering.
Even organizations like PETA recommend against pestering restaurants about small details and trace ingredients, and I couldn't agree more: it makes vegans look nitpicky and rude and makes it harder for most people to want to make baby steps in the direction of reducing animal products.
My advice to new vegans: be easy on yourself and others. I'd rather see ten people who reduce their animal products consumptions by 90% than one extremist who does 99% and alienates everyone else.
Would you care if bits of human carcass were mixed into your food? That's how I feel about the cows that you all are murdering.
It matters because a burger that smells and tastes like real meat because it got into contact with meat is not vegan.