Thanks for getting back to us Linewbie.
Yes I agree, there's a lot of time involved in creating a plugin like you have. I also think you should get paid for it. But there's a difference in giving the impression that it's free when it's not, and sell it like others are doing with their plugins.
Look at WordPress Affiliate Pro. Also a great plugin. The current price for that plugin is $97. It's a lot of money but people are willing to pay for it because it saves them so much time. If that author told you that it was free but every once and a while put his affiliate ID in the place of yours, it would be stealing.
The same goes for All In One Adsense and YPN. Most people have no programming skills and therefore it would be pointless for them to even look at the code you wrote. All they know is that they can use it for free and that's it. While they are working very hard to attract visitors etc., they're unaware that 8% or 15% or whatever are taken from them.
Having a option in the control panel to turn donation of is nice, but it has to be made clear on the download page that such a function is there, because we know that the default function is going to be On. The sad thing is that there are thousands of other clueless people who is and will still run the old version and lose money on a daily basis.
You took a lot of money from a lot of people. No matter how you look at it, it's wrong.