In today's environment ANY policy is...no matter how silly the claim.
I don’t think it ought to be a “get on the scale and we’ll charge you by the pound” type of thing - if you are larger than the average seat, then they ought to have the passengers who are larger pay to sit up in the wide first class-type section or pay for 2 seats before boarding. It should NOT have to do with tall or short. It’s a matter of whether someone is comfortable in the seats sitting next to one another. If someone purchases a regular ticket and the check-in agent sees that they aren’t going to fit by looking at that person, they charge them then for the upgrade or for 2 seats and there or don’t allow them on the plane.