I think you're absolutely right. There's nothing fundamentally wrong with giving a t-shirt, or even a gift card for a t-shirt, if it's properly communicated. But it has to explicitly be something like, "Thanks so much for this. I'm terribly sorry that we can't give you a proper reward, much though I'd like to. As a small token of my appreciation, I hope you'll accept a t-shirt as my way of saying thanks. Since I don't know your size, here's a gift card you can use to obtain whatever size and style you prefer." If it had been done like this, I think it would have gone over fine. There might be some grumbling about how a large company "should" have a bug bounty program, but not the sort of attitude we've seen.

As best I can tell, the gift card was offered without comment. And while the intentions were good, the lack of such a message meant that it was taken badly.

I guess the lesson is, if you're offering only a token of appreciation, make sure the recipient knows you know it's only a token.