While they absolutely could / should have rewarded him with more than a lousy t-shirt. I suspect they aren't keen to encourage a bunch of random people to try and poke holes in their security with the expectation of a cash price.
You can either reward your own citizens with large cash prizes, OR, you can reward Russia/China with your data since they'll gladly poke around for free.
This is being penny wise and pound foolish.
Most humans also need a full-time job to survive. If I wouldn't have to work a job to live, I would have more free time for good Samaritan pen-testing for the government if that would pay my bills instead.
In some socialist European countries, artists get subsidized by the state to create "art" instead of working. Why can't we do that with pen-testers? Sit around at home on UBI and look for zero-days in government infrastructure?
(BTW, maybe check on the origin of "Good Samaritan". His saintly disinterest in any sort of personal gain was the whole point of that story.)
The t-shirt is actually kinda nice in that regard. Of course, it would go better with stuffed euros.