If you're going to put something out there, free or otherwise, it's your problem to make sure that it's fit for my purpose.
You're not entitled to my money, and I'm not entitled to your content. The difference is, though, I didn't dangle my wallet on the Internet with the intent of wasting everybody's time.
If someone builds a website and puts it out there with advertising, they're saying "Hey everyone, feel free to enjoy what I've built without paying me for it, because I've got these other guys paying for it. Just look at the ads and click on them if you find them interesting, ok?" When you come along with AdBlock, you're making use of the website without allowing the author's arrangement with the advertisers to work. Ultimately, the advertisers will stop paying, and the website must either disappear or become paid-subscription only.
Of course, most people don't like ads, and the more obnoxious they are the more we dislike them and want to block them. But I think we'd like Xanadu less, where every website would have micropayments required to view it. I know that I'd rather put up with ads, especially the less obtrusive ones.
Keep in mind that, at this point, microsoft was pretty much nothing...
Here was a guy that built something with the intent to sell it, and people said "too bad". Of course he was upset about that.
Now, should he have been philanthropic enough to have just given it away to the community? Maybe, but it doesn't really matter...he coded it, and he did so thinking he would be able to sell it.
I equate those that copied BASIC then to the people that use adblock now. No, they're not really harming the author, but it is a bit discourteous. I think he was just trying to illustrate that "C'mon, guys...I worked really hard on this...I'm not suing you, but just please buy it instead :)"