This is redunant to the answers already out there, but since this is one of the top search results on Google, I thought it should have a reference to where this is officially documented.
Enjoy and Happy Splunking!
I expect he's likely still under the trial, and wants to convert early or doesn't know that it will convert on it's own. Cervelli explains it well here:
To paraphrase, under the license page there should be an option to convert from the trial to free.