Locking people up over marijuana may be stupid, but it’s not medical. I didn’t elect him and he needs to keep his non-medical opinions separate from his job. From global warming to the pandemic, medical experts and scientists taking political sides has lead to disinformation, hesitancy to do what should be done, stonewalling or even outright censorship of the facts.
Clearly the basis for banning drugs is the harm they may cause to the health of both their users and others. A medical expert's opinion is relevant in assessing harm. It's not like he weighed in on net neutrality.