Under the feudal system, rights to one’s person were alienable (swearing fealty to a lord was irrevocable) while rights to land often weren’t (a feudal lord couldn’t sell his estate, as it belonged in perpetuity to his heirs) – the exact reverse of the rights system that most libertarians advocate. Forbidding the alienation of rightfully alienable property is as much a violation of property right as any other.
Now even libertarians who defend IP generally regard it as alienable; so the current move by the IP lobby to attack the voluntary alienation of IP rights should be something that pro-IP and anti-IP libertarians can agree in opposing.
An argument one sometimes sees for the inalienability of IP is the consequentialist one that if IP is treated as alienable then creators will be exploited by big companies. It’s certainly true that under the current IP system, the chief beneficiaries of copyright tend to be not the original creators but instead large publishing and recording companies; and making IP inalienable is one way to address that. Yet inasmuch as IP, whether alienable or inalienable, constitutes both protectionism and censorship, it remains objectionable on both rights-based and consequentialist grounds. A better solution to the exploitation problem is to replace IP-based business models with ones that secure compensation to creators in nonviolent ways.