United Airlines (motto: Passengers Slow Everything Way Down) has decided that since they've already begun charging for baggage, snacks, drinks, window seats, aisle seats, reservations, keeping the plane in the air, and landing at the exact same airport that you selected as your destination when you purchased your ticket, the only way to gouge passengers is to make them buy two seats if they are overweight.
As obesity continues an epidemic march through these fifty states, passengers are growing larger and seats are remaining the same sad width. United sees this as the perfect time to squeeze cash from the portly, because what are us tubbies gonna do? Stop eating so much?
Other airlines (Southwest...hint hint) has a similar rule but with a much different approach. If you're too large for a single seat, they attempt to move you to a row with an open slot so that you don't spill over into someone else. Southwest only resorts to billing customers for two seats if the plane is full and there are no other options.
United, on the other hand, will swipe your credit card twice even if the plane is half-empty if you require more than one seatbelt extender. It's hard to imagine why people have such difficulty maintaining airline brand loyalty.
In some countries (Canada?), obesity is considered a medical condition and treating the obese differently than everyone else is prohibited.
At least Canada gets it.