So wait, this guy was smoking some weed (I assume) in the middle of a con and got banned. Why is this even questioned? Whether you agree with smoking weed or not, he committed a crime during a PyCon event.
If cannabis use is legal in the state, there is no prosecutable crime. Federal cannabis drug law is not enforced against cannabis users, only those who sell it and thereby more directly touch on interstate commerce.
If breaking unenforced federal law is ban-able behavior, then better ban everyone.
I think the unfortunate thing is that we all break federal law every day. (Most of us at least violate the Computer Fraud and Abuse Act. Ever made a GET request potentially in violation of an API's ToS?) Scary thing is, the feds actually prosecute under the CFAA.