You're correct, there are two functions in Exchange which were written by the team at Shopp, and if you bothered to look at the code you're referencing you'd see that it is, in fact, attributed immediately before the function. By honest mistake, we actually only attributed one of those functions, but we've corrected the issue and credited both to him. We did not try to hide these attributions or our use of the GPL code. In fact, we went out of our way to reference any GPL authors whose code we used and modified. A clear example can be seen in our db session code.
I appreciate your concern, and because of your post we were able to attribute an additional function that we honestly missed. Our intention was not to misrepresent ourselves or our plugin. But this is clearly not a GPL violation as you've claimed. If you have any further questions, please let us know.