They'll probably have legal grounds to do so, and here's why:

Unless United is using an objective standard to determine who must pay the extra charge, they will be discriminating. So will it be a BMI analysis, a tape measurer for waist size, a scale with a height/weight ratio chart, or maybe a demo seat in the airport for fatties to sit in for determination as to how much flesh is "spilling over". Who will be responsible for this . . . the gate agent who already has 20 people in line screaming at her?

That's the main reason schemes like this are public relations nightmares. Does United have the chutzpah to instigate an objective test and apply them ruthlessly to all of their passengers? We'll see, but I think it's a fairly desperation move to raise cash.

One other thing . . the vast, VAST majority of Americans are already overweight, with over 25% in the "obese" category. If this policy is objectively applied, as it MUST BE, United is going to alienate a big portion of the American public.

No one likes sharing their seat. This policy seems like a good answer . . for about 2 seconds. If this is the best United can come up with, they're in serious trouble.

From a purely business standpoint, this policy is sheer lunacy. Even if they do manage to make this ill-conceived notion to stick, they'll just be driving away a substantial portion of their customer base who don't want to be humiliated by being evaluated and judged by a United ticket counter agent in public.

They'll just go elsewhere.