Theme Editing

Hello All,
I have a question on themes....

I have seen quite a few sites state that they used a certain template as their "base" and then began editing it to fit their needs. Many of these sites have removed the credit at the bottom of said theme for it's original creator. Does this mean that with X amount of changes a theme can become "yours" in a sense? Or perhaps they have made some sort of monetary arrangement with the original creator?

Thanks
Devon