You are right there are two views, but they are reconcilable.
I think the problem mostly lies in conflating "token of gratitude" with "gratitude". The token is a cheap thing, and has some small monetary value. The gratitude itself is a much more valuable thing, for both the giver and the receiver. Reporters are motivated by doing the right thing, and perhaps getting some recognition for their contribution. The recognition has two parts: recognizing cleverness (you caught something others missed), and recognizing honor (doing the "right thing", reporting it to the affected party rather than exploiting it yourself).
You can't put a price on cleverness and honor. And if you try to, and you come up with $12.50, then you're an idiot. The only thing you can do is recognize it, and show gratitude for the help.
What the OP tried to do was show gratitude and recognition with a token, and some recipients confused the token for the gratitude. It doesn't sound to me like the giver was confused! E.g. he wasn't saying "Here, let me pay you for your time, eliminating the need for us to recognize your cleverness and honor for the cost of a T-shirt." Now you might say that the token was poorly chosen: there is a reason people invented trophies, items that are totally worthless other than as tokens of accomplishment. It's because the recipient will NEVER confuse a trophy for the price of the accomplishment behind it.
Personally, I think a cool little useless trophy would have worked better, and certainly no gift certificates! Being named and honored somewhere on the Yahoo website would be great. Giving a small cash prize, like $1000, would be a classy move.
The bottom-line remains: there was nothing fundamentally wrong with the T-shirt-as-trophy, apart from the (apparent) risk of recipients confusing it for T-shirt-as-payment.