I will probably get flamed for this but here goes anyways. When you split the meal you are using twice as many condiments on the burger bar that you didn't pay for. The bar is included for the person that bought the meal. Your kinda stealing to save money. Alright go ahead and flame away
While I see your point, is it called a burger bar (my whole point hinges on this, so disregard the rest if I'm wrong about the name)? If it's just called a "condiment bar", then there is no reason why I can't reasonably presume I'm entitled to condiments for my bun. Unless they explicitly indicate somewhere as to the purpose of the bar, I think spliting a double cheeseburger onto a second bun and putting condiments on that is a 'reasonable use' of the condiment bar...
Bringing my own bun, splitting the burger, and using the bar--probably not so reasonable of a use... I guess I'll just pay the extra $1.59.