I think it's not so much national borders so much as how the rights to books are sold. Normally, if you sell a book to a publisher, the rights to sell the book in the US and Canada are one deal, and the rights to sell the book in the UK, Ireland and the Commonwealth are a separate deal. (This is why you tend to get different US and UK editions of books.) Some authors are only successful in selling their books in one region.

But therefore, it's impossible to make ebooks free in the two different regions, because often they've been sold to a completely different publishing company than the one who publishes them in the other region. Does that make sense?

I suspect a VPN would get around that, but I've never tested it. As I say, my objection is less to the fact that there are region specific ebook giveaways, and more that Tor.com refuse to state this fact plainly when promoting it — it's all just 'free ebook of the month!', which leaves a very sour taste in the mouth of this non-American reader.