The processors are soldered to the board in the MacBook Pro and Apple has offered BTO options for faster processors. Also look at the BTO graphics options for the different iMacs. Some models have different GPU's that are soldered to the logic board. I don't think that soldering is a barrier for Apple.
Agreed, it's not necessarily soldered vs removable. As you've pointed out, Apple does it on plenty of other machines with their CPUs/GPUs.
It most likely has to do with the volumes of the machines. Apple knows that they move sufficient numbers of each product to justify having separate logic boards for differing configurations. With the Air, I don't think the volume is there. If they went ahead and did it anyway, they might be left with unmovable stock, which costs Apple money. That extra expense would be passed on to the customer. So they don't do it.
As it stands anyway, there are 2 logic boards used for the air: the 1.86 and the 2.13. If they added a 4GB RAM option to the top end only, there would be 3 configurations.
Personally, I think it might make sense just to drop the low end model all together, and only have a single logic board configuration: 2.13/4GB. Then give an option for a 128 or 256 GB SSD. Leave the pricing where it is now ($1499 base, $300 for the 256GB upgrade).