Chomsky is talking about institutional factors. He typically admits he is affected by them the same as anyone else. What kind of readership can a public domain book get? How would publishing it be funded? If it was a success, another publisher could just begin a print run. Old public domain books manage to get published, but, they have the advantage that they don't need to be advertised, have shelf space speculatively bought, etc. etc.

Chomsky also doesn't believe we should have regular property (at least not in the "rights" sense), but he still owns stuff.