So, register the plug in. If you've done 20 MP3 renders, it's time to register. It's free. Just open the MP3 dialog, click the register button, you'll be done.
Sony does this because just to "give away" the registered version means they pay royalties for the encoder, when a lot of users will never use it. So, you get 20 freebies, and then you register, which is a freebie except it costs time.