Hello Caitlin,
Just for the sake of clarification:
All models of the first 2 versions of the iMac G5 had the ability to be wall-mounted.
The only models that cannot be wall-mounted are those with the iSight camera built-in.
But, I did have another thought as to why they might have elected to leave this option off of the recent machines.
I know from personal experience that the iMac G5 would get very, very hot.
And, it was obvious that this led to a lot of their repair costs.
So, with the redesign that included the iSight camera, they did change the entire interior design to attempt to make it run cooler.
Now, it's possible that they wanted to assure that these machines had adequate ventilation behind them. By having the foot installed, it can only be placed a certain distance from the wall. You cannot place it any closer than the base of the foot will allow.
But, with a wall-mount, you could place it flat against the wall, or 1 inch from the wall, or 5 inches from the wall, or even a foot from the wall.
Not knowing what kind of wall-mount adapter you might choose, it is probably safer to prevent you from doing so.
I know when I was shopping for wall-mount adapters for my system, that I was only able to find one within 100 miles of me that would support the heavy weight of the monitor and allow it to sit more than 1 inch from the wall.
Most of the adapters designed for heavy weight are designed flat to hold the weight securely.
Arms that extend out away from the wall have some instability. The arm I used was designed for the weight. But, even that one needed some custom re-inforcement to keep the screen pointed at me and not the ground (and it was a horizontal arm if that gives you any idea).
The arm was only about 5 inches long, and made of solid steel. But, because of the weight of the machine pulling against the brackets sole source of support (the plate at the end of the bracket), the machine would tilt down.
I beefed-it up. And, it worked. But, it was definitely not ideal. And, it was my only option if I didn't want to place the computer too close to the wall.
I did some experimenting, and found that even at a distance of 2 or 3 inches, the ventilation space behind the computer was still inadequate for cool operation.
So, you really need about 4 or 5 inches of space. Every inch makes a huge difference in how cool the air around the computer is. And, the cooler the air around the computer, the better the computer can cool itself.
I would suspect that these factors probably were major considerations in the idea to get rid of the wall-mount adapter.
I agree it was nice. But, since you cannot count on the user to insure proper air-space, they probably chose to go the safer route.
Hope this helps.