IIRC, the foundational issue of this is not unique to WA, many states(if not all?) have use tax laws that indicate tax is to be paid on items such as internet purchases or where tax was otherwise not collected. The difference being that because it goes through source that can be authorized to collect the tax they actually enforce it. For example, when I moved, even though I had owned my vehicle already, they inquired as to the tax I had originally paid on it at the DMV, if the tax I had paid had been less than the local tax rates I would have had to pay the difference to the state. (Fortunately, or unfortunately, the sales tax in Chicago is something like 9.25%, so I didn't have to pay)
I think the reason there is no issue with taxing firearms is because it's not a special tax for firearms, it's the general sales/use tax applied to all goods in the state of WA or wherever, albiet with unequal enforcement, when compared to stuff you buy on amazon or some other internet retailer.