Selectively quote all you like, but the Court never said that copyright infringement never equals theft. Your own citation illustrates that, noting that the former "does not easily equate" with the latter, but not that the two never equate. In fact, the Court struggled with that notion well into the opinion, noting that copyright infringement ""fits [] awkwardly with the language Congress chose," but may fit nonetheless.

As a result "of the ill-fitting language" (note that the Court still is unsure of the conclusion here) it is forced to resort to "the history and purpose of 2314." Here, it discovers that the National Stolen Property Act was an extension of the Motor Vehicle Theft Act, whose purpose was to eliminate barriers preventing law enforcement from foiling criminals moving stolen property across state lines. The Court reasoned that because copyright was a creature of federal statute, making borders meaningless, perhaps it wasn't intended to be covered by the statute in question.

The Court has to go further still to justify its conclusion. It analyzed the legislative history of the criminal copyright statutes to conclude that Congress has legislated very carefully when it comes to applying felony penalties to copyright, and ultimately concluded that it may not have intended the harshness of this statute to apply to the facts in the case.

And finally, the Court magically reads into the statute a physical limitation on what can be transported, even though those words never appear in the statute or its legislative history.

Ultimately, the only guiding legal principle in this case is (1) to qualify for prosecution under the National Stolen Property Act, things being transported across state lines must be physical goods; and (2) in a choice of two plausible statutory meanings in a penal statute, the Court requires a clear and definite statement in order to choose the harsher alternative.

And the decision is not as "very clear cut" as you make it. The dissent makes a powerful argument that the NSA should have applied. It noted particularly that:

"Virtually every court that has considered the question has concluded that 2314 is broad enough to cover activities such as Dowling's" and that the majority's decision is against the weight of national authority.

It goes on to point out that "[t]he statute makes no distinction between tangible and intangible property."

The dissenting Justices also note that, "[t]he statutory terms at issue here, i. e., "stolen, converted or taken by fraud," traditionally have been given broad scope by the courts."

And they conclude by noting that merely because Congress provided some penalties in the Copyright Act, it does not preclude the government from prosecuting under other federal statutes, ending with a damning statement that "[m]any courts had used 2314 to reach the shipment of goods containing unauthorized use of copyrighted material prior to the enactment of the Piracy and Counterfeiting Amendments Act."

Clear cut, it is not.

As a final note, copyright infringement has been equated with theft/stealing for hundreds of years. Beginning in the 1600s, pamphlets likened infringement with shoplifting, mail theft, purse stealing, highway robbery, burglary, theft from a hospital, and most notably, piracy. Our Copyright Act, for instance, preempts “all legal or equitable rights that are equivalent to any of the exclusive rights within the general scope of copyright” According to the case law, courts have found the following causes of action preempted by the Act - conversion, theft of services, and theft of satellite signals. So to say that an equation that has been commonly used for centuries by individuals, courts, legislators, and governments is somehow wrong based upon a few cherry picked quotes from one case is simply disingenuous.