I heard UA is slowly transitioning to a minimum fare code level in order to earn 100% of the FF miles.
I believe DL and Co operate under this scheme in the US, I know that BA has long since been this way. Reports show business travelers are willing to book a higher fare code in order to maintain their status and earn 100% of the miles, not that they are paying anyway!!!
I have noticed the more recent UA FF promotions all require a farecode higher than W.