If it's a free theme, can this be found on a site offering premium paid themes with an affiliate program?
Especially if it's a theme that you bought, why not use the affiliate program to grab some extra cash?
I'm always clinking the credit links on themes that are attractive to me to see if there is a new theme I can use for a site.
I don't know if it's newbies not knowing any better when slapping up a site, or if some just don't care about it. But why not utilize the credit link as another income stream?