I'd like to see an economist explain the rationale behind the first-sale doctrine applying to IP on physical media but not if it's not tied to physical media in the US (note that the EU currently does approximate applying it to non-physical media). I have a really hard time seeing a reason for that.

I can believe that the doctrine of first sale shouldn't be a thing. And I can believe that it should be a thing, and should apply to all forms of media. But applying to one but not the other seems like a pretty hard sell to me.

Physical copies degrade over time, whereas digital information may not. Works in digital format can be reproduced without any flaws and can be disseminated worldwide without much difficulty. Thus, applying the first sale doctrine to digital copies affects the market for the original to a greater degree than transfers of physical copies.

Okay. But...so what? Why do we care whether the market for the original is affected? If that were a factor, wouldn't we object to the legality of making backups? Wouldn't we treat more-durable forms of media differently than less-durable forms of media, or take into account the decay in value of the IP itself that lives on the media?

Like, I could understand maybe an argument that permitting a vendor to restrict physical media transfers of IP is economically desirable but simply isn't enforceable, ergo we're better off without a lot of halfway attempts to restrain it. But I've never seen it explained with that as a rationale.