That's correct, but with the perpetual license, there may be 4 computers active and 2 concurrent users.
I suppose it's because CC takes over the control of the CS6 installation, when the Adobe id attached has a valid subscription. I'm guessing that using CS6 with a different id will solve the problem. However, as the CS6 install is "contaminated" with CC licensing, you need to completely uninstall, clean-up (Adobe CC Cleaner app) and reinstall. I never had a CS6 license, so I do not know what changed against CS5. I hope however, that your license is not attached to the Adobe id.
If that is the case, you need to contact Adobe Customer Care (Contact Customer Care ) to solve that problem first.