Slightly different but last year my mate had a card confiscated - he couldn't make a game so sold it on to a lad he knew through someone else. It was the league game immediately after the Chelsea semi final. The lad failed to return the members card saying it had been confiscated by a steward because of a report of racist abuse by someone in the same seat at the Chelsea game.
Now me and my mate sensed bullsh*t, but thought it might have been the lad who used the ticket playing up resulting in the ticket confiscation. My mate missed a few games whilst trying to work out what happened with the club, eventually the club replaced his card with no real explanation of what happened.
After all that I suspected if a steward being clever to their own gain, and having read your post wonder if it is the same.
Steward picks a game where the members card would need lots credits. Spots someone who doesn't look like they use that ticket every week (they can stand out working out where their seat is). Steward then abuses his position to confiscate the ticket. Steward then can abuse his position again to find out what seat the ticket is for and sell the card on, or use a mate to sell it on.
At first it sounds far fetched but if you have the steward has the balls to try it its pretty straight forward to pull off, even misleading a police officer to help you. Theres unlikely to be comeback as the member card owner has broken rules so is fearful of being in trouble.
So I suspect a steward is flogging your cards on for the Man Utd game, and did similar with my mates card last year (although was in the kop).
I don't believe for a minute its a genuine policy by the club. Such a hap hazard way of picking people, and with no clear idea of what has happened after. If it was the club they would have properly contacted your or my mate with what has happened, why they have done it etc.
And if the club really wanted to stop this there are easy fairer solutions:
1) the cards that have a different person each week requesting a duplicate at the ticket office 'as they forgot their card.' (this happens)
2) the away tickets that are touted on 3rd party websites for huge prices.
These are easy to pick out. The club could let people watch the game then take action against the person involved. For example (1) I'd warn them, and for (2) I'd confiscate the season ticket. Make an announcement on the website and everyone is happy as the touting is being addressed, more away tickets become available and a few people move up the season ticket ladder.