Well, the actual message says "This item is not Available for this Region." in red letters and I don't see it until I've placed either the CD or Download version into the cart and go straight to the shopping cart page with the full cart contents.
For example,
this title is completely unavailable to Canadians, which is a shame, since it was one I'd have liked and they're having a 50% off audiobook sale (apparently we also get VAT deductions which knock the price down by a further 17%).
By contrast,
this other title is available to me as a Download, but I'm blocked from buying the CD, which seems mildly weird, and
this third title I can buy both Download and CD, assuming I wanted to pay 9.99 pounds sterling in shipping for the latter.
As for having no buy buttons, some of the titles seem to be only available in either CD or Download, not necessarily both.
Though if you are indeed missing out on any buttons entirely, maybe the entire US is just not covered by their site, which seems like an oddly specific and surprisingly thorough geo-restriction given that the rest of the world seems to be okay.