1. You forget that the average American needs at least 4 Big Macs to be satisfied and the extra $2.68 will mean he won't be able to drive to McDonald's. The consequences will be devastating for gas-station owners.
2. I don't own McDonald's stock and so I wouldn't be able to benefit from their increased profits
You forget that the average American has plenty of fast food choices, and might even cook their own food is pushed far enough. There's a reason fast food places rarely raise prices.