The plaques on the seats have been there for some time, probably been removed and replaced every time they've upgraded the seating in the directors box. We know this as during the clips of the events prior to the Everton game, after his death, you can see the plaque on the seat where he'd sat.
It's quite easy for them to leave the plaque on, it would do no harm leaving it there and they don't have to leave the seat empty either. The free season tickets don't bother me at all, even if they are no longer in the directors box.
For me it's just another move for the club to far, it's minor, it's petty and all they had to do was keep something small on a seat. If the club started to slight Fergie the way it has slighted Busby over the years, I'm sure the reactions of many fans would here would be different.