I think of donations as an expression of appreciation. There's nothing wrong with that. You get a bonus at work if you excel at your job, too.
Of course you do not donate to all and every addon you use, but maybe the ones you really like/need/love and like to keep updated. After all quite some addon authors only pay the damn subscription fee to write, test and maintain addons. At least that's what I did before I quit a year ago for the previous 2-3 years.
Besides, you should think more out of the box. There are other countries, with different living conditions than in the US. A few bucks are way more worth for someone living there. I'm not saying one should dedicated all their time to wow addons and expect to make a living from that, because they can't otherwise. But people making some extra money out of their hobby in their free time, isn't a bad thing.
That being said, it's not like people are donating a lot of money on addons, anyway. Like Phanx said, the most money would be in adds and subscription fees. And you could argue about those ways of making money more than about a donation.