I would guess that listing commercial providers on the CentOS site could expose them to legal liabilities given the litigious nature of the U.S. so don't know if this is a consideration.
I can't speak for others, but I would guess that there are quite a few people/organizations on this list who provide commercial support for CentOS and other *nix distributions without being listed on CentOS's site or other sites.
We have been providing support for a variety of Linux systems since 1997 staring with Caldera, then SuSE, and now most of our clients are running CentOS on current systems. We have been supporting a number of clients since 1984 running various Unix systems including some old SCO OpenServer 5.0.x systems.
The critical issue for the client is that they have confidence in the organization that does their support, not that the organization is a big company such as Red Hat, IBM, etc. I ran into our oldest client today in town, a company that we have worked with since 1984, and is still running applications on SCO OpenServer 5.0.6a. We have other clients that we started working with as far back as 1988 when they were running SCO Xenix, and now have systems still running SuSE 9.0 Pro (it still runs liunx-abi to run SCO OpenServer COFF binaries), and have a couple of CentOS 5.x systems which are hosting the Caldera systems.
Bill