It was their plans that changed, not yours. You booked your space for them, prepared for a stay of that length and potentially turned away other guests. And you have chosen a cancellation policy ahead of time that fits you. There is no reason at all to offer a refund. In fact, most hosts would ask you NOT to do so since it trains guests that the cancellation policies and other rules really don't mean anything. If they didn't ask you, don't bring it up. If they did, you can refer them to airbnb to cancel the remaining days as per your policy. Let them contact airbnb, don't get involved.
That said, there may be emotional reasons you might feel like it - did a guest's parent just die? Or their home burned down? There is no reason to be a heartless jerk if it is something compelling.
Also, you may want to think about your flexible cancellation policy for your lovely space. Is it really ok with you that someone could cancel at any time (or 24 hours before, I forget which) with no penalty at all? You might be attracting guests who have more flakey planning and might wind up empty when you could have had at least a day and a half of earnings is you had done moderate. Just a thought!
Also - just saw that airbnb editted your listing description this way- "We have hosted Apple, Stanford and (SENSITIVE CONTENTS HIDDEN)...". You may want to remove that other company's name (I am guessing the big search engine company!) so it doesn't look so secretive or saucy!
Good hosting to you!