Yes, some can control their catalogues to some extent and some can't, or at least have a very large say over what gets released or how/when - especially after their initial contractual obligations are done. Let's remember that in Zep's case, they had/have their own record company (Swan Song), although this upcoming concert is going to be a "tribute" to Atlantic guru Ahmet Ertegun.
I've seen reports where Jimmy says there may be other shows after the one in London, while Robert has said there's 'no chance' of that.
As a part-time drummer myself for many years, I'm certainly not on the side of the record companies. But I also know how musicians can get apathetic and uninspired to create new material, or not interested in gigging anymore - until it comes down to making the car payments.