Copyright/powered by WordPress

Hello,

I am considering using WordPress for a small business site and am not sure I understand the terms of the GPU license.

Most websites that use WordPress put “Powered by WordPress” at the bottom. Is this a courtesy or a legal necessity?

Also, if one used a free theme, would one be legally required to put “Theme by xxx,” etc. or is that a courtesy?

What if one edited a theme and then used that?

I realize that creators and authors deserve credit for all their hard work, and I have no intention of doing away with that. I am asking because I want to know the legal requirements.

Thanks!