Well I didn't take it that was as I don't consider GWDs to be anywhere near Rolls Royce, Picasso, Rembrandt, etc. nor is this a painting, it's a business. A business who's selling their products/services, not someone else's.
I also can't compare a site to a movie/TV show, etc., it's just not the same thing. They are giving credit to EVERY person who worked on that set, not just one person.
Does a site owner give credit to the website coder? To the copywriter? To the shopping cart software or vendor? To the PHP programmer? There can be many people who take part in creating a site, not just the GWD.
As for books, the writer gives credit to whomever he wants to give credit to, the book designer or editor can't sneak it in there without the author/publisher finding out about it & I highly doubt they ask the author, "can you give me credit in your book please."
If you think it's supposed to be that way, then tell me why you don't see it on all or most sites?
No offense. Glad your clients are doing it for you, I wanted to know what the norm is just in case I was missing something since the last time someone told me it's a no no.