Resolved: St. Thomas would not condemn internet piracy.
Exhibit A: SCOTUS itself, in accord with the tradition of US copyright law, distinguishes between copyright infringement and theft (cf. Dowling v. United States). Copyright infringement is thus an offense against positive law and not against the natural rights of the copyright holders.
Exhibit B: We turn then to St. Thomas on positive law. "Now laws are said to be just, both from the end, when, to wit, they are ordained to the common good--and from their author, that is to say, when the law that is made does not exceed the power of the lawgiver--and from their form, when, to wit, burdens are laid on the subjects, according to an equality of proportion and with a view to the common good." (ST IaIIae q.96 a.4)