As Griffin explained it, Userful is mostly what you would probably call a system integrator. That is, they aren't maintaining their own distro; the software they deploy seems to vary depending on the contract (hence Educational Linux instead of Userful Linux in the case of Brazil). So I don't know who actually wrote the multi-seat X component they deploy, but the impression I got was that the company is not primarily software engineers.
As it stands now, Multi-seat X can be done by nesting multiple X servers (eg, with Xephyr) -- which is slow -- or independently configuring an Xorg server for each front-end (which is no doubt painful, and probably not easy to do on the USB terminals). So my guess would be that whatever solution they have rolled into Userful Linux simplifies the configuration of maintaining 10 Xorg servers on one host machine, in a location without an Xorg hacker on staff. It may even have required special work for the hardware support; but that's speculation.
In any event, my point was that they aren't pushing their X solution as a product and may even be licensing it from someone else, because it can't be done effectively in free Xorg. If that changes and a future X server can natively handle multiseat, I would expect them to switch over to that -- no matter what the situation is, not maintaining your own X server software saves considerable time and resources.