AFAICT, the reason is that there's a big jump in the median fine if you're going >15mph over the speed limit vs <= 15mph over. Economically-speaking, it's not worth the police officer's time to pull somebody over who's "merely" 10 over, because somebody going 16+ over will appear in a few more minutes of waiting.
Interestingly, the state website says the official "maximum" fine bump occurs at 11+ mph over. I suspect they wait for 15mph to rule out the plausible cover stories "I was only going that fast to overtake the other car" or "new here, I didn't realize the incline was so sharply downhill with nobody in front of me".
You probably wouldn't get pulled over for going 80-82 in Washington (i.e. they people passing you probably have their cruise controls set for 80), but it might be less enjoyable of a drive. You have to pay more frequent, closer attention, because you're overtaking people more frequently.
Ironically, California dives deep into "low fines for speeds under 15 over". the under-15-over fine base is a meager $35! For 16-25, it jumps to $70, which is still paltry.
It's more like $300-800 depending on how much you are over the limit. There's a bunch of extra fees they toss in.
Googling it doesn't bring up any kind of official source, but several websites for law firms that corroborate that range.
Usually when I have cruise control set to exactly 75 and I'm in the right lane, I'm very often passed by people who are probably doing 80 to 82 mph.