You get that for an awful lot of chips or software which are thought to have "dual use" to aid inadequately defined nefarious things.
On most orders for electronics parts, even if it's European distributors shipping to me, in Europe, I'll have to click some boilerplate where I assert to not make the chips available to terrorists on the Denied Persons List of the US department of commerce, which is conveniently available in ASCII...
The same applies when buying microcontroller starter kits, like the MSP430 for school.
That, and clauses about not using it to controller missiles. Which is admittedly more applicable than the weapons of mass destruction (exaggerated) clause in iTunes.
Ah the beauracracy.
I dug up this text on their site: https://www.sparkfun.com/support#IntExportRestrictions, which says:
[...] These are products designed for civilian use, but that could be used for devious purposes in the wrong hands. [...]
So, I guess that tells me there are weapons or other "dangerous" machines that have UARTs. Who would have thought that, huh? :| Really dubious restriction, but I guess they don't have a choice.