It actually isn't. Cannabis is an exit herb, meaning it is not physically addictive.

Using cannabis to treat the issues you were using drugs to soothe is a substitution, yes. The medicinal benefits assist in quitting, comfort and more. The mistake is to assume that simply using cannabis will remove the original use for the drug. (in this case nicotine)

Quitting a drug is easier and craving free when the underlying issues creating the 'addiction' are dealt with. Once the physical addiction is also broken with the assistance of cannabis, the need for cannabis diminishes rapidly.

Solve the medical need another way, the cannabis desire evaporates and becomes a 'pleasure' thing instead of an 'addictive need,' thing.

The prevailing 'institution' on addiction is crumbling, in the face of cannabis being used as an exit herb.

Glad I could help. :D