Cheating is not an issue at all. People can cheat better when there is no source code - by decompiling or so, they can still find exploits. However, server admins are powerless against that, as the game company no longer supports the game, so there are no further patches. With the source code released, on the other hand, the community itself can fix those issues. That said, game companies might be reluctant to release source code for this reason just because they're unaware that it's not an issue at all.
Removing references? If they had those in their code to begin with, then they were pretty poor coders... There is no reason to put that kind of information in your source code. It would be the equivalent of scribbling that information on a company's financial report - it might be used only internally in the company, but there is no reason for you to put that information there for others to read it to begin with.
Reused code and 3rd party IP are indeed the most important reasons not to release the code (Epic Games don't release their code for the former reason). That, and laziness.