I doubt it's a large crowd. It only take one person to take a sign. In my experience, when people know something old is about to be replaced (and possibly discarded), they are more likely to consider taking it than they would otherwise.
I would doubt that the ATC would broadcast their information for that very reason. Have you seen what people pay for AMC signs at the auction? A few went for $1500-$2000 this last year, and they weren't AT summit signs.
And with the increase in sign vandalism I have seen in the past number of years, I'm not giving the benefit of the doubt as much as I used to.