Violates the 14th Amendment as well (income is considered property in WA)
For a tax to not violate the 14th it have to be applied against all wage earners in the state at the same rate of taxation.
ARTICLE VII
REVENUE AND TAXATION
SECTION 1 TAXATION. The power of taxation shall never be suspended, surrendered or contracted away. All taxes shall be uniform upon the same class of property within the territorial limits of the authority levying the tax and shall be levied and collected for public purposes only. The word “property” as used herein shall mean and include everything, whether tangible or intangible, subject to ownership. All real estate shall constitute one class: Provided, That the legislature may tax mines and mineral resources and lands devoted to reforestation by either a yield tax or an ad valorem tax at such rate as it may fix, or by both. Such property as the legislature may by general laws provide shall be exempt from taxation. Property of the United States and of the state, counties, school districts and other municipal corporations, and credits secured by property actually taxed in this state, not exceeding in value the value of such property, shall be exempt from taxation. The legislature shall have power, by appropriate legislation, to exempt personal property to the amount of fifteen thousand ($15,000.00) dollars for each head of a family liable to assessment and taxation under the provisions of the laws of this state of which the individual is the actual bona fide owner. [AMENDMENT 98, 2006 House Joint Resolution No. 4223, p 2117. Approved November 7, 2006.]