There are 2 problems: First, Margherita is tomato and mozzarella, with optional basil garnishment. Period. It's the recipe as it is and as it has always been; if you add an ingredient that heavily belongs to another recipe, you are profoundly altering the original spirit of the first. Second, garlic is not considered good with mozzarella. As an Italian, I find it really unfit due to the cultural taste I'm accustomed to.

A foreigner ordering margherita with garlic is violating our sense of taste and altering a template that doesn't allow such distorsions.

As a counter example, you can add without problems an egg on a margherita; it becomes a "Bismark pizza", but it's fine since the egg plays well with tomato and mozzarella. Any modification that is an enhancement, rather than a distorsion, is alway welcomed.