This really is not so simple. There remains a debate as to whether this issue should be something that is reserved to the states under the tenth amendment. It is true that right now the majority of the Supreme Court in the raich case held that the federal government has the right to regulate pot, but two justices dissented and ultimately, in this country the law emanates from some combination of the three branches which have struggled for power over time.
Several states have chosen to legalize medical marijuana and a few recreational. Two justices on the Supreme Court believe they have the right to do so. The executive branch has explicitly agreed to leave those states alone. And he arguably has the right to do that.
In my view, it is therefore legal in those states that authorize it. It is also true that tomorrow president Cruz or trump or whoever could change Obamas policy on this, or president Clinton could liberalize it, but as of right now, the president has agreed, there is a constitutional argument that the federal law overreaches the state,s sovereignty, so to say it’s illegal under federal law is not exactly right.
Ps. The federal government has been notoriously anti pot. One reason for the current laxity is similar to the Jewish law concept that you don’t make a law the klal won’t comply with. The fact is that there is widespread noncompliance in states where it is still illegal,, it is probably less damaging then alcohol, and if the states are going to permit it, the Feds are going to have a real hard time enforcing without state cooperation.