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.
cbsnews.com/news/two-states-legalize-pot-but-is-it-really-legal/