Washington has a gross receipts tax, and no corporate income tax.

"The Business and Occupation Tax (B&O tax) is basically a gross receipts tax, measured by a specific tax rate against the value of products, gross proceeds of sales, or gross income of the business. Unlike a tradition income tax, there are no deductions allowed from the gross receipts or income of the business.

The rate of tax depends upon the type of business that you are conducting. The major B&O business classifications and their related tax rates are:

 B&O Classification
 
Rate 
Retailing 0.00471 
Wholesaling 0.00484 
Manufacturing 0.00484 
Service & Other Activities 0.018 

Maybe because the tax is so tiny compared to the one SEIU wants to impose?

While you are comparing, Belleview has a 9.5% sales tax.  Here are the exemptions. SEIU's tax has no exemptions.  

 http://dor.wa.gov/Content/FindTaxesAndRates/RetailSalesTax/Exemptions.aspx