Theming WordPress and other popular scripts is one of the best features anyone has ever thought. But like most good things, there is always someone that will exploit something to his benefit taking advantage of someone’s lack of knowledge. Unfortunately, nowadays it has become quite common for someone to buy a theme for $40, tweak its fonts, colours and images, and then resell it while presenting it as if it was his own work. For me this is just… stupid! Use a theme, tweak it as much as you possibly can. But don’t go out saying that you made this. Because there is a huge difference in code manipulation and code creation!
As for the stupid part… most of the people that take pride on work done by others, have no idea how theming works and so they can’t even properly hide a theme’s identity. That said, using What Theme, anyone can find out if their favourite website is using a custom theme or if someone tricked them into using a beautiful theme but not unique!