True: "[W]hat these states are trying to do is consider Amazon's independent affiliates a physical presence for Amazon in that state."
Also true: "[H]ow [a customer] found their way to Amazon and made their purchase isn't relevant."
The problem: The two statements are directly at odds with each other.
Amazon doesn't currently charge sales tax to things shipped to California under the exemption that they don't have a presence in California and therefore their commerce is strictly inter-state.
California has a new law which effectively creates a de-facto presence if someone who lives in California can be an agent for a sale of something from Amazon. (those are the affiliates) and it is the intention of the state to use the existence of those 'agents' to close the tax loop hole that Amazon is currently exploiting.
So when it goes into effect, Amazon terminates its relationship with all affiliates who live in CA(or have their checks sent to California), thus keeping the loop hole open for them, and causing pain to some segment of California voters who may (or may not) be motivated to tell their representative to reverse their stance.
You, up in Oregon, don't see a change except that if you bought from someone in California through Amazon before, well you won't be able to do that now. You'll have to go directly to their web site.