So it was the battery that caused the problem?
Ostensibly—but the regs, even as posted on United's own website (I linked to them in the article) are quite clear that it should have been allowed. You'd have to willfully misread—for example, to assume that the Segway miniPro is not a type of Segway but a type of "collapsible wheelchair", or that it doesn't have computer-managed lithium-ion batteries but old-fashioned "dumb" pure lithium batteries—to get to a reading where the battery's energy capacity was even at issue.
Sorry, it looks like a fun thing to use.
It is! It's been so, so useful to me, but yes, it's a lot of fun too. :-)