For example, some states hold that their residents have an "expectation of privacy" in their garbage, meaning that the government must have probable cause to search garbage. Some states hold that traffic "check lanes," where entire lanes of drivers are stopped and checked by the police, are unconstitutional. The United States Supreme Court has held that the Federal Constitution does not recognize an expectation of privacy in garbage, and that traffic check lanes are constitutionally permissible.
States can only add to your Federal constitutional rights -- they cannot pass constitutional amendments that restrict the rights you receive under the Federal Constitution.
A state court's interpretation of the state constitution can differ from an interpretation of an overlapping federally protected right. The variance may arise from a number of circumstances, including textual differences, different legislative histories, and disagreement among courts about the correct interpretation of constitutional language. For some, independent state interpretation of state bills of rights has the benefit of allowing states to be laboratories of experimentation for new or different legal doctrines. Moreover, a state court may be better able to provide stability and clarity of law than a distant federal court.
The problem here is that Scalia used differently worded State constitutional guarantees to try to interpret the US constition. These differently worded provisions only act to obfuscate the proper inquiry.
Another no-no, but Scalia was coming up with his new right.
The problem is that there already was a unanimous decision in US v. Miller that provided this guidance to the interpretation of the Second Amendment:
In the absence of any evidence tending to show that possession or use of a 'shotgun having a barrel of less than eighteen inches in length' at this time has some reasonable relationship to the preservation or efficiency of a well regulated militia, we cannot say that the Second Amendment guarantees the right to keep and bear such an instrument. Certainly it is not within judicial notice that this weapon is any part of the ordinary military equipment or that its use could contribute to the common defense. Aymette v. State of Tennessee, 2 Humph., Tenn., 154, 158.
The Constitution as originally adopted granted to the Congress power- 'To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions; To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress.' U.S.C.A.Const. art. 1, 8. With obvious purpose to assure the continuation and render possible the effectiveness of such forces the declaration and guarantee of the Second Amendment were made. It must be interpreted and applied with that end in view.
UNITED STATES v. MILLER, 307 U.S. 174 (1939)
The obvious answer here is that Scalia had a new interpretation which was without legal basis or preedent that he wanted to justify.