Question is, why are they farming atlantic salmon on the pacific coast?
You've missed the point. Of course they do it because it makes business sense. That's the whole point of a business.
What remains a mystery is why on earth would they be farming an invasive specie right in the ecosystem they are a threat.
That's far from being the point, and it doesn't answer anything at all.
I mean, I assume this "environmental nightmare" happened on a fish farming business located in Washington. After a brief googling, it appears that Aquaculture companies in Washington are required to apply for a commercial license.
http://wdfw.wa.gov/licensing/commercial/misc_additional_perm...
Why on earth would a commercial license be granted to a business that involves introducing invasive species on a protected ecosystem?
My sister is a marine biologist for a fish farming company (not the one involved in the article). My understanding from past discussions with her is that there have been attempts to farm some of the species of pacific salmon, but they have not yet figured out how to get good yields.