Not only rappers can smoke a lot of grass. Its use does not cause critical harm to health. Many countries have already begun the process of liberalizing legislation in terms of decriminalizing production and distribution - the United States, Australia, Belgium, Czech Republic, Canada, Uruguay, Portugal and others. So there is no objective reason for this to end. Quite the opposite - everything is just beginning.
The question is, why won't they be jailed, or why isn't their health deteriorating? If 1, then for a track in which, for example, Wiz Kalifa or Pharaoh read about the use of marijuana, you cannot jail, because you need proof. In the USA and Europe, such performers are very loyal; Snoop Dogg, for example, can safely smoke weed at his performances. And the sale of drugs is much tougher (you can remember Yung Trappa, for example).
If 2, then marijuana is practically harmless to health (it is already legalized in Canada). So they can smoke as much as they want.