A lot of the geography and alt-history of Shadowrun do show its roots as being written mostly by guys from the northern part of the USA, some years before the web became a thing. Do remember that most American schools don't teach a lot of world history or geography.
So the first reaction of people from many places in the world is "what? That makes no sense because X, Y, Z, reasons.". (Believe me, even in Canada there is lots that makes us wonder, and we were right across the border).
But ultimately I find the most fun approach to these issues is to accept them as writen, then determine for your campaign what _really_ happened, and why, and let players discover some of it in play.
You can come up with your own reasons. For me with resopect to Iceland, they chose to join the TPA despite having no cultural affinity. Why?
- the aura borealis often came with intense, disruptive, mana storms that also tended to cause strange spirit phenomenon. The shamans of the TPA seemed to have ways to deal with it, which Iceland needed.
- in the resource rush, the seas were heavily overfished, with other European fleets disregarding Iceland's normal fishing zones. This created intense anti-European feelings in many Icelanders, and a need to find new places to fish. The TPA was willing to offer up some of the deep sea zones that its people did not normally reach.
- Being part of even a weak state offered some sense of protection when the world was going to hell.
- iceland new that with the most advanced infrastructure of the TPA that it wold become the defacto banking centre and so forth.
So a marriage of convenience was formed. Whether it was popular with everyone was doubtful, but enough voted for it, in some way, on one day, that it happened (kind of like Brexit in reverse)