The question DOES state that Clapton is your next-best alternative activity, which indicates that there is nothing you would rather do than see Clapton. So, using that part of the question, you wouldn't want to give away your ticket. And, because it has no resale value, it is worthless any other way if you don't go. So, the question appears to be valid to me.

Opportunity cost is not to be confused with value. In my brief reading of the paper, it seems like that is where people got tripped up. Still, it is disturbing to me that so many educated economists got the question wrong.