They have a right to do what they feel is best for their respective businesses. It's important to remember that while some states have legalized marijuana for recreational use, the federal government still classifies it as Schedule One drug under the Controlled Substances Act. Maybe they want to wait for the Federal vs. State issue to be resolved before they start accepting ads.
Even though big internet companies can target ads to specific locations, they are still reluctant to allow marijuana advertising. More: gigaom.com.
Even though I may dislike this decision, it's within their purview to take on whatever advertising they want.
If they feel marijuana ads are not representative of what they do or who they are or what facade they present to the public, then that's their right and more power to them.
I do believe that if marijuana were made legal tomorrow we would see a slew of ads in every medium possible, that is, until the the government stepped in and told everyone what they can and can't say about the products they offer.
It's their business. They can accept or deny whatever advertising they choose. I'm certain there are other areas besides marijuana that they refuse. Why do you single this product out? If someone finds their policy regarding marijuana (or any other product) advertising inappropriate, they have the right to not use these services.
Companies have views and morals to uphold, especially Google who have a high reputation that they want to keep that way. They are allowed to turn down anyone they see not going to give a good look for their company. Just like business can not employ people due to body modifications such as tattoos and piercings or shops refusing to serve someone due to the way they've been acting. They have a large say in the world, and therefore need to keep "clean" so to speak.