Major problem with drawing the conclusion that marijuana use caused the increased risk, which of course the study did not do. It is just as likely that the people were using pot to self-medicate because they felt sick! So it's just as valid to say that increased risk for stroke, heart failure, coronary artery disease and sudden cardiac death cause medical marijuana use.
More and more sick people are using it: It is only normal that some of those sick people get sicker and die. Most marijuana use only alleviates symptoms. It does not cure the underlying problem.
Another problem: The study subjects "were discharged from hospitals in 2009 and 2010, when marijuana use was illegal in most states." That means they included all the people who were using illegal marijuana and who knows what other drugs. Several other illegal drugs are known to cause heart problems like amphetamine, MDMA, and injecting anything cut with insoluble substances.
I think they need to design a better study now, which will be much easier with legal recreational use in some states. They can get a large sample of users who are healthy and don't use other drugs and use a similar sample of health non-users as the control.
