Such characterizations are based mainly on Sotomayor's participation in a January decision that said state and local governments are not bound by the Second Amendment. But because it was arguably compelled by a series of 19th-century Supreme Court precedents, Sotomayor's conclusion does not necessarily signal an anti-gun bias.
The appeals court cited Presser v. Illinois, an 1886 decision in which the Supreme Court said the Second Amendment "is a limitation only upon the power of congress and the national government, and not upon that of the state." That decision, which relied on the 1875 ruling U.S. v. Cruikshank, was followed in 1894 by Miller v. Texas, which said "it is well settled" that both the Second and Fourth amendments "operate only upon the federal power."
Nowadays, of course, federal courts routinely ask whether state or local governments have violated the Fourth Amendment's prohibition of unreasonable searches and seizures. According to the Supreme Court, that guarantee, along with others in the Bill of Rights, applies to the states by way of the 14th Amendment. The trio of 19th-century gun cases came after the 14th Amendment was ratified but before the Court began reading it to incorporate restrictions that had hitherto applied only to the federal government.
Now that the Supreme Court has said the Second Amendment protects an individual right to arms, applying it to the states, along with the First, Fourth, Fifth, Sixth and Eighth amendments, is the logical next step. A footnote in D.C. v. Heller, the 2008 case in which the Court overturned the District of Columbia's handgun ban, suggests it is leaning in that direction: "With respect to Cruikshank's continuing validity on incorporation, a question not presented by this case, we note that Cruikshank also said that the First Amendment did not apply against the States and did not engage in the sort of Fourteenth Amendment inquiry required by our later cases."