The issue is collecting sales tax for items bought outside of their state. Interstate commerce is a federal jurisdiction. If they collect sales tax in Washington state, it's because they have an office there.
You're obligated to pay sales tax on items purchased out of state, by annual remittance. Almost nobody does it, but if you're ever audited (by your state), they can ding you if you don't. Amazon is plenty big enough to be specifically called out by cash-strapped state governments for, effectively, encouraging people to evade the law.