@Jeffro
Wow - Mark's answer is in reponse to my comment, but I never noticed it. I've just left a response, which includes:

Additionally, it's just occurred to me that one of my plugins (not yet in the repository but hopefully one day when I clean it up), requires an external link to Yahoo! because it uses their currency rates api.
The plugin is free, it's GPL, but it makes a call to a Yahoo! api and they require the link. Are you saying that plugin can't be added to official repository?

Anyway, I still think I'm entitled to add a link if I want to. In over 10,000 downloads, I've only ever recieved one $15 donation. I'm not paid for my efforts, but I'm not allowed to ask for a link in return for providing free software? Man, that's harsh...

Note, I always provide a way to turn the link off (except in the Yahoo! case as it's required) and I don't add a link if it doesn't make sense.