Hello,
We do that to prevent members from changing the domain to something else once approved. We've had members get a Great.com approved and then change it to a NotGreat.com in the title. In all of our other marketplace sections, you can edit your listings at will.
However, you can request that we update the title or post of your Top Domains thread for you. We're happy to assist with that. We just need it to go through us so we can make sure that NotGreat.com doesn't end up in Top Domains.
Hope that helps,