The airlines have rules for these situations, however, they will never enforce them.

The other issue is that the airlines are continually decreasing the size of the standard Y seat in an effor to cram as many people into Y as possible.

I guess this is what happens when airfares continue to decrease to the point of absurdity (airfares have remained steady during the past 10 years, however, when you factor in inflation, they have actually gone down).