Some historical context may be helpful here: For a very long time Washington state had a highly regulated, tiered program for selling liquor. Very recently, this was privatized. The marijuana legislation that passed, I-502, very closely apes this structure - which kept financial interest in the production, distribution and retail of liquor separate - hence the separate taxation at each "tier".

The explanation here could best be summed up as "the devil you know".

Further, as with liquor, this decriminalizes personal possession without requiring tax stamps. This tax structure is in place to regulate and generate revenue from the production, distribution an retail of marijuana - not necessarily the consumption.