Quoted: Why do I see quite a few pictures of people with side rail mounted optics on a side folding stock AK? I was told that you can't fold the stock without taking off the optic.

This it true.

It is kind of a pain in the butt.

I guess the idea is that when the rifle is stowed the stock is folded and the optic is tucked safety in the pouches they usually come with. Otherwise the stock should be out and the optic attached.

One shouldn't get into the habit of firing without a stock anyway, but I agree that it's a pain and I would have liked it if the stocks would have been designed to fold to the other side while still allowing for access to the charging handle and safety lever.

On another note... why does it cost like $150 to get the Arsenal SLR-107FR over the Arsenal SLR-107F? It seems as $150 is way too much for a side rail.

It is. They do it because most people want the version with the side rail.

I'm tempted to get an Arsenal SLR-107F with one of those new Larue red dot/iron sight.

Those Iron Dots are pretty fragile compared to most Eastern Bloc optics that attach to the side rail, but of course you can fold the stock with one attached.