The GPLv2 section 6 says to those who redistribute another person's GPL-licensed work:
Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein.
The central question, then is whether threatening non-renewal of a support contract, based upon distribution of the GPL-licensed program, constitutes a "restriction" on distribution. It is not obvious to me that it would be, but I cannot rule out the possibility entirely.
In particular, the recipients of the GPL-licensed code have 100% of the rights detailed in the license, and the set of restrictions upon the recipients' right to redistribution is strictly those requirements imposed by the GPL. The distributor has erected no additional legal barrier for those recipients to exercise their right to distribute their copies of the GPL-licensed code. If the recipients choose to redistribute the work, the upstream company (e.g., Red Hat) does not have the ability to take any legal recourse, nor have they ever claimed the ability to do so.
They have, however, threatened to take practical recourse by refusing to renew a support contract. Their right to do so stems from the existing support-contract between the two parties, not the licensing relationship. The question is: when a contract includes a term that is activated by the exercise of a licensed copyright right (where the license grant itself is external to the contract), will a court view that as, in fact, a term applying to the scope of the license? This sounds like a very nuanced legal question, and even if we could get an answer, it may vary by jurisdiction, depending on the legal norms of contract law and licensing law.