Here's a thought. The gillnetters are used to protect the tribal fishery. Without the gillnetters you would have one fishery that can and does fish selectively with little mortality to the incidental and non-target stocks. If the tribal fishery were the only "kill 'em all" show going it might be politically harder to portray ones self as conservation minded. So, the non-Indian gillnets provide cover.
I suspect that if the tribes were forced to fish selectively that there would be a myriad of legal challenges such as what is the superior law-ESA or treaties. And, the one that should concern WA the most; how many salmon do we need to ensure the tribes have access to (Boldt II).