I don't think it's that people can't bear to crop out their work. It's that there's a school of thought that says the thumbnail should be a mini version of the entire image, or it's deceptive. There are many people who refuse to click on an image with a cropped thumbnail, or pass on artists who use them.

I try to use the whole image when possible. Not everyone has broadband, and given the problems Rosity's servers are always having, I want to give people an honest preview of the image before they click. But sometimes, the image just doesn't thumbnail well, and I'll crop it.