This because during installation there is no possibility to fill in a product key. That must be done after installation. Strange thing, because with System Center Configuration Manager (ConfigMgr) 2012 it can be filled in during installation. When you don't fill in the product key, an evaluation version (180 days) is used by default.
To upgrade OpsMgr 2012 from evaluation to Retail version use the following commands:
1) Start Operations Manager Shell (PowerShell)
2) Enter "Set-SCOMLicense –ProductID <product-key>"
3) Choose Yes or press enter for default
4) Reboot the OpsMgr server or restart the "System Center Data Access Service" (!)
After that start the OpsMgr 2012 console, choose Help > About, and you will see Version 7.0.8560.0 (Retail) now.