Think of the thousands of dollars contractors spend to bid on projects they never get. Or think of the lawyers that sue for a percentage of the award and lose the case. Or think of the health care practitioners that care for patients that never pay. There is a ton of work being done without compensation out there, and the only one who knows if it is worth it is the one doing the job.
Obviously it would be better to be compensated for the travel. But if you feel the job is a good fit and provides an improvement worth the risk, then pay your own way and be happy with that decision whether you get the job or not. Just because you don't get the job doesn't mean it was the wrong decision to spend the money or take the risk. It was still the right decision, it is still money well spent, just need to look at the long game. By making investments in yourself and your career appropriately, you will be better off over time. Absolutely that justification can be abused, and is all the time. People justify luxury cars to impress clients, justify expensive lunches and fancy clothing, etc, etc. Pull down the curtain, clinically assess if the job is worth the travel cost to interview based on your available money, the improvements the job would bring, whether it would be satisfying, etc. And then if it is worth it, spend the money without regret or looking back at those sunk costs.
If a job was in high demand, an employer might use the fact that they don't reimburse for interview costs as a way of limiting the applicants to those who are really serious and dedicated to the idea of that job. Or they might just be cheap bastards... :D