Originally Posted by Shareholder

Actually it's UA who's the stingy one in this respect, though I suspect it is also an agreement between certain carriers not to offer miles when the other carrier has similar restricted fare categories. If you are a member of another FF program and have your flights on UA credited to that program, UA must pay that program to purchase those miles on your behalf. If you pay $1100 for a W fare between NAmerica and BKK/SIN, this means UA must pay that program to buy its miles. Given the round trip would generate 20K base miles, at even a penny a mile, UA would have to pay up $200. That's almost 20% of what you paid for the ticket.

So given such margins, it stands to reason UA would only want to buy those miles from the other FF program when you've bought a high-fare ticket and not a low-fare one.