Travel reimbursement for interview after accepting another position
Both parties are at fault for not communicating the terms of the reimbursement clearly.


It is reasonable for the university to want to reimburse only interviews that were actually attended, and to not want to bother interviewing applicants who have already declined. 
It is also reasonable for the student to want to get reimbursed when they have personally expended funds in good faith. 


It's a bit difficult to even say what the university's policy for such things should be: 


Not paying these expenses encourages applicants to lie, go through with the interview, and waste everyone's time in addition to money.
Paying these expenses encourages people to get an interview, cancel, and then use the free tickets for a vacation (doubt this happens all that often, but it's possible, and can be a major source of scandal). 


Regardless: in the real world, I think it's rather unusual for institutions to reimburse interview expenses for an interview that never happened. Given this reality, applicants are the ones that have the burden of establishing the reimbursement terms before shelling out their own money.