This is just a guess, but here's my 2c. I would think that, in terms of £s per hour, a popular guest will make more at a signing desk than at the photo booth - so economically that would mean it would make more sense for SM to have him signing on the busiest day.
Secondly, the fact that the photoshoot is on the Sunday means that those who want to have a photo done will need to be there on the Sunday - meaning it draws in additional attendees for the less busy day.
Or people who can only go one on the Saturday and specifically wanting a photoshoot, will say "oh well, I might not bother going at all and just wait and hope he comes back again", it could work both ways.
The amount of people who come to that decision will be small, in my view. It's a business, and SM have to concern themselves with maximizing their return, and that means get someone signing on the busiest day. An autograph and a hello takes what, 30 seconds? Sure, it varies - Patrick Stewart churned them out like a madman, whereas others take longer, and have a brief chat. But that's a reasonable estimate. A photo takes a lot longer. Get someone signing on the busiest day - basic.
On top of this, no matter what arrangement SM put into place someone will lose out. Presumably your idea would be that there should be a photoshoot on both days? This would take him away for a couple of hours on the Saturday, and a lot of people would lose out on Autographs. As Queen Sindel says here, it *is* a double edged sword, and to be perfectly honest in the spirit of PT Barnum SM cannot please all of the people, all of the time.
To be perfectly frank, if you can't make it on the Sunday, that's unfortunate, but everyone has stories like that. I've missed key guests because I couldn't make it to the particular event - I would have loved to have met Michael Ironside, and Michael Biehn, but I couldn't make it to that event. In their cases I suspect they'll be back, but still, it's annoying. But SM cannot make arrangements that suit everyone.