On a themes site they stipulate, in the terms and conditions, that their themes inherit the GNU general public license from WordPress. I believe that this means that they can be copied and modified.
However the themes are being sold. Shouldn’t they be free? Could someone enlighten me.
The GPL absolutely does not preclude selling code, for cost. In fact, the terms of the license explicitly allow for selling of code.
The GPL merely allows the end user, whether he received the GPL-licensed code for free or for cost, to use the code however he likes, to redistribute the code, to modify the code, and to redistribute the modified code – provided that any such-distributed code be licensed under the same license.