I have no idea at all whether this is true or not, but it's a belting piece of recieved wisdom nontheless.
I was once told that because of the burger ingredients, including masses of fat, sugar and salt, it should be classed as a cake or confection product. The pickled gherkin, as a vegtable item ingredient, prevents this, and thus prevents VAT being chargeable on top of the price, and obviously a pr disaster for Ronald, too.
Thinking about it, this is possibly just about mad enough to be true, in the US, and global homogenisation means now we all get the gherkin.