This quote sums it up:

"Not every airline is so protective of their most loyal customers. United, for example, packages up the nice treatment its elites receive – the dedicated airport check-in counters, special security lanes and priority boarding – and sells it off to anyone who can cough up as little as $9 per segment for the privilege."

Hardly a coincidence that when they wanted to find an airline that does not protect its loyal customers, they choose UA and, by contrast, when they want to find an airline that does protect its loyal customers, they choose AA.