I think my title is more descriptive than the one MSNBC chose, New law allows loaded guns in national parks, but their title is accurate, if incomplete. I rather doubt that this will do much to noticeably change the crime rate in National Parks, for the same reasons I've noted before. In Arizona especially, this primarily affects concealed carry permit holders, who are an exceptionally responsible and law-abiding group of people. There really isn't any other reason to take loaded weapons to Grand Canyon National Park than concealed carry for self-defense, since hunting is not allowed in any National Park.
Really, this law just allows state law to take precedence. The MSNBC article says that the legal climate shifted abruptly in 2008 with District of Columbia v. Heller, but really this all was a long time coming. The balance of legal opinion in the US seems to favor increasing individual autonomy wherever possible, and despite the more extravagant claims of some, constitutional jurisprudence really does focus on the Constitution. Thus we cannot really consistently limit 2nd Amendment freedoms more than the freedoms guaranteed by the other Amendments. It has always surprised me a bit that there is a cultural divide between the ACLU and the NRA on this issue, because they are really making the same argument.