Why not ? There are more than a few sketchy pot stores among the thousands of licensed dispensaries spread across the 23 states and DC where medical marijuana has been legalized. Fortunately, you can just have your weed order delivered. All you need is a green card, the Internet, and cash.
There doesn't seem to be a compelling reason to prevent marijuana delivery businesses operating in states where the drug is legal. In many places, individuals can have alcohol delivered to their home and these businesses operate without disruption to their local communities. In the case of medical marijuana, where the drug is being used to treat chronic pain, one can also see how helpful such a service would be to people that use the drug.
In states where pot is legal, delivery of pot should also be legal. Some people smoke weed purely for its medicinal value. Those people should not be forced to go to a pot dispensary just because they want their pot. For recreational users, surely some of them drive around to get their pot. If states allowed delivery, there may be fewer people driving around impaired.
Legalized marijuana is available at state-approved businesses. This is the way it should be. A person purchasing marijuana should have to prove they are old enough to purchase it and should follow legal guidelines. Delivery of marijuana would open up many legal questions, such as whether it was being delivered to minors in a household. Regulations need to stay fairly strict.