The Gnu Public License (GPL) allows you to use copyright work (the GPL'ed code) as long as you follow the license.. if you don't follow it, you lose the right to use the copyrighted work (the GPL'ed code). That's all really.. they now stopped distributing code which included GPL'ed code, so they're allowed to keep the rest closed. (They could instead have chosen to open up and continue using GPL'ed code, but they chose the other way.)

Of course, all this closed code from Nokia is not to their advantage IMO. Some of it is due to other vendors (TI stuff etc), but what about e.g. that original email application? For example. (Bits were open, but not enough to use for coders.)