Well, I must say I do not totally agree...
Releasing GPL is the right, and legal, way to do it.
It's one option among others.
...you really need to release the plugin under the GPL licence.
I would say : "It is recommended" or "I recommend". The fact is that it is not needed.
You can distribute your plugin under the license you want. If you want to invent your own rules (or own license), you can. The GPL license given with WordPress allows you to do it.
In short: No, you do not have or need to release your plugin under GPL. The fact is that the open source mentality wants you to do it that way and that's what is recommended. But that is not an obligation.