You can see it as an insultingly low payment or as a bigger than usual thank you. If you expected nothing, the shirt is a pretty decent thank you. If you expected to be paid, the shirt is an insult. Clearly the people who bitched about the shirt expected to get paid. So they had already put their work in the "paid" category and then complained when the pay was low. It sounds like a lot of other people didn't expect anything and put their work in the "free" category and were (at least a little) pleased to get an unexpected thank you.