GPL is copyleft, they're mainly doing additions and modifications to code that is GPL. They have to publish their code. I believe they try to be sneaky and hide how they add it but it's all open source.
Terminating a contract when a customer redistributes the code seems to be a violation of the GPL to me, but I'm not a lawyer.
I also heard they obfuscated some code and removed comments to make things more difficult, before the RedHat acquisition of CentOS.