Federalism and Marijuana

In the Bessette/Pitney blog, there is an article about the Justice Department's decision not to prosecute marijuana growers and suppliers that comply with their states' laws. In it, Kleiman states that "the regulation of medical practice is a state and not a federal responsibility." On what does he base that assertion?

On the election of Senators

Article 3, Section II of the Constitution says that each State legislature is in charge of choosing its federal Senators. One rationale for this is provided in Federalist #9, which claims that it makes the states "constituent parts of the national sovereignty, by allowing them a direct representation in the Senate." The 17th Amendment reversed this, however, making Senators subject to a direct popular vote.

Why was the 17th Amendment passed? What might the Founding Fathers say about it? How were the politics of the Senate changed as a result of it?