There are multiple interpretations of the Interstate Commerce Clause. By some interpretations, States do have limited rights to regulate commerce with other states. Also, there seem to be additional interpretations of the law for state-owned services (See the paragraph on "In United Haulers Assoc. v Oneida-Herkimer Solid Waste Management Authority (2007)".
See the following site a good summary of some of the debates.
http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/statecommerce.htm
"The Commerce Clause is a grant of power to Congress, not an express limitation on the power of the states to regulate the economy. At least four possible interpretations of the Commerce Clause have been proposed. First, it has been suggested that the Clause gives Congress the exclusive power to regulate commerce. Under this interpretation, states are divested of all power to regulate interstate commerce. Second, it has been suggested that the Clause gives Congress and the states concurrent power to regulate commerce. Under this view, state regulation of commerce is invalid only when it is preempted by federal law. Third, it has been suggested that the Clause assumes that Congress and the states each have their own mutually exclusive zones of regulatory power. Under this interpretation, it becomes the job of the courts to determine whether one sovereign has invaded the exclusive regulatory zone of the other. Finally, it has been suggested that the Clause by its own force divests states of the power to regulate commerce in certain ways, but the states and Congress retain concurrent power to regulate commerce in many other ways. This fourth interpretation, a complicated hybrid of two others, turns out to be the approach taken by the Court in its decisions interpreting the Commerce Clause."
