Pot's already legal up here in Washington, the law went into effect a couple days ago but it's gonna be quite a few months before dispensaries start selling it.

IIRC it's taxed pretty heavily just like alcohol and cigarettes are, and while its not illegal to have it on your person (so long as its under the designated weight limit) if you are pulled over with marijuana in your system you can get a DUI which is tricky since pot stays in your system for a lot longer than booze does.
You could smoke on monday and get pulled over on thursday for speeding and if the officer suspects you smoke (you have a pot leaf sticker or are listening to reggae) you're probably gonna get hit with a DUI.

You also gotta be 21 before you can buy it legally and you can't just have it out in the open when you're walking through town, much like booze

Rather than argue about why its illegal, why don't you give a good reason why it should be legal?

"I was surprised at these findings. I thought we were going to find some small negative affect on pulmonary function from marijuana use. But we didn't. At the level of marijuana that most marijuana users smoke, we did not find any appreciable negative consequence on pulmonary function on marijuana use."
Mark J. Pletcher

Although as with any inhaling abusing the product will cause long term damages.