I've heard the term in relation to airlines catering to a type of customer or market... Midwest Express could have been considered a boutique airline catering to mostly business destinations and a few leisure, esp with the amenities- 2x2 leather seating, gourmet meals, costing a little above the legacies, etc... now that they're Midwest Airlines and have the Saver Service off-shoot, I wouldn't consider them a boutique airline. Another could be the both EOS and Maxjet Airlines operating out of JFK
, operating business/first configured 757s and 767s respectively to London. In relation to Alaska... I'd say its because they are in many cases the only airline to the northern destinations they serve, and cater to that, especially having the combi 737-200s to carry passengers and extra cargo; but then again they have extensive operations on west coast routes in general as well. Hope this helps!