I've seen some say, "makes sense," when that information was tweeted over the past week by various sources. But I have to say this is the first I've heard this. I searched through various TechNet and other Microsoft posts about Windows 10 servicing and could not find a previous mention about this Surface exemption.
Other Windows PC makers haven't imposed similar attempts to limit the ability of users of other Windows 10 devices to make use of LTSB (that I know of, at least).
Microsoft's official reason for the Surface LTSB policy:
"LTSB prevents Surface devices from receiving critical Windows 10 feature updates and certain non-security servicing updates. Therefore, LTSB is not supported as a suitable servicing solution for general-purpose Surface devices. As a general guideline, if a Surface device runs productivity software, such as Microsoft Office, it is a general-purpose device that does not qualify for LTSB and should instead run Current Branch (CB) or Current Branch for Business (CBB)."
Microsoft is not saying that those using LTSB on special-purpose Surface devices will be unsupported. Anyone using Surfaces to control medical equipment, point of sale systems, and ATMs are not running afoul of Microsoft's guidance.
The LTSB was designed for specialized systems, but was available to all Windows 10 Enterprise users as an option, as far as I had read and understood. Microsoft never publishes feature updates through Windows Update on devices running Windows 10 Enterprise LTSB; instead the company makes available a new LTSB release every two to three years. Users can install these interim updates or skip over releases for a 10-year period.
Microsoft's goal with LTSB from the outset was to keep the number of Windows 10 machines on that branch relatively low. Officials didn't want LTSB to become a crutch -- a way for those business users nervous about regular, ongoing feature updates to be able to avoid them.
For those wondering, I asked Microsoft officials what "not supported" specifically means in the case of general-purpose Surfaces running LTSB and haven't heard back yet.
Update: A spokesperson reiterated that "LTSB prevents Surface devices from receiving critical Windows 10 feature updates and certain non-security servicing updates. Therefore, LTSB is not supported as a suitable servicing solution for general-purpose Surface devices." Microsoft officials seem to think this answers the question as to what "not supported" means, but I am still confused what Microsoft will or won't do if a customer decides to run LTSB on a general-purpose Surface device.
The company has nothing to share beyond that statement, however, a spokesperson said when I asked again.
Update No. 2: Microsoft also just posted on TechNet recently "Considerations for Surface and System Center Configuration Manager," which reminds users that device drivers and firmware updates for Surface hardware comes via Windows Update. I'm wondering if this is related to Microsoft's "not supported" edict. Maybe Microsoft is just reiterating that those on LTSB won't get these kinds of updates to drivers and firmware, so their devices might not function/function properly, moving forward? Just a guess on my part.