I'm not sure if what I found online is correct, but this from Yahoo seems to indicate that the onions are added as the patty is being grilled. If so, I would think a customer could just request no onions.
The new burger formula will include softer, toasted buns; warm, melty cheese; and onions that are lightly caramelized because they're added to the burgers while they’re still on the grill.
Onions are bad for a lot of animals. In birds (parrots), the metabolites attack the respiratory system (or so my vet tells me).
As an aside, what the $*#@ is "melty cheese"?