I think you're absolutely right: this is very much a philosophical matter, rather than a legal one. There are plenty of reasons to oppose SOPA, after all, both philosophical and practical. But to return to and expand upon the philosophical side of things, here is an excerpt from a good review of Patry's _Moral Panics and the Copyright Wars_ that deals with some of these issues:

http://ip.jotwell.com/bill-patrys-war-on-copyright-rhetoric/

"Patry’s book focuses on the role rhetoric and metaphor have played in legislative and public debates relating to copyright lawmaking, and how they were instrumental in the continuous expansion of copyrights in both duration and scope. In particular, Patry explores the function metaphors like 'property' and 'piracy' have played in the lawmaking process. Because abstract concepts such as copyright law are difficult to grasp, people tend to rely upon and fall prey to imperfect metaphors, especially when they are used repeatedly by industry lobbyists.

Patry is no fan of a common stock of well-worn copyright metaphors, and he attacks one after another: expressive works as bearing a personal relation to their author (the 'copyright as giving birth' and 'orphan works' metaphors), ownership of copyrighted works as an absolute, unlimited title (the 'property' metaphor), copyrighted works as the singular product of genius (the 'creation on a clean slate' metaphor), and the money to be earned from copyrighted works as the rightful reaping of what an author has sown (the 'agrarian' metaphor).

In Patry’s view, these inapt metaphors serve only to distract us from the real issue, which is how to structure copyright law - a form of economic regulation - to best promote authorship, learning, innovation and progress. And while Patry documents various rhetorical tricks played on Congress’s floor, he also laments what has been missing from the debate: 'in my 27 years of practicing copyright law, I have never seen a study presented to Congress that even makes a stab at demonstrating that if the proposed legislation is passed, X number of works that would not have been created will be.'

Central to Patry’s argument is the structural role that 'moral panics' have played in this process, namely the furtherance of a public state of hysteria respecting illusory threats emanating from 'folk devils'. Such panics often concern youth behavior and new technologies, both of which are not well understood and portrayed as a danger to core social values. Such panics are manufactured in order to capitalize on public (over)reaction to such (imaginary) threats in order to advance economic and social regulation that entrenches incumbent interests. Moral panics contain the following elements: the suggestion that a dire state of emergency is being brought about by a new threat to social order; the suggestion that swift action must be taken in order to prevent imminent social harm; the presentation of false and misleading data to lawmakers regarding the magnitude of that harm; and the suggestion that preemptive action - in this case action protecting copyright industries - serves the national interest. As one would suspect, Jack Valenti makes guest appearances throughout the book."