I've been doing some digging through older news articles, and it really seems like the signs were there for quite some time now - BBRY knew that the US carriers would not be first out the gate.
Here's some reading - November:
BlackBerry 10 will get full carrier support, RIM exec says | Mobile - CNET News :
August - focus here is entirely on enthusiasm of Canadian carriers. Though later re-bloggings of this story made it sound like RIM said that ALL carrier were enthusiastic, it's clear that when the news first came out they were talking about Canadian carriers:
RIM Touts Carrier Enthusiasm for BlackBerry 10 | News & Opinion | PCMag.com
I also recall with 100% certainty that early last year, Thor made some remarks that very specifically indicated that US would not be among the first to receive the devices -- but I can't recall enough of the exact wording to find it.
My point is - in spite of discussion about carriers dragging their feet, or BlackBerry getting burned on the launch because carriers weren't ready to go... the signs have been there for the last 12-14 months. Every indication I can find (and recall) seems to say that this was in the cards (and their plans) all along.