>Due to a quirk of Washington's constitution, we can't charge income tax.

More precisely, there is case law from 1933 (Culliton v. Chase) in which the State Supreme Court held that income is property and thus can only be taxed at a flat 1%. There is an argument that the court got it wrong in Culliton by misunderstanding the prior holding in Aberdeen Savings & Loan Assoc. v. Chase from 1930, as laid out here. If that argument holds water, legislative or initiative action to instate an income tax may prevail.