Honestly, I only just donated now. But charity is about giving, not about making a purchase.
If the map was going to be hidden behind a wall like a kickstarter or patreon reward, WOTC would have used that format for the charity. But that's not what WotC's trying to do: they want to encourage donations to reach those goals (and now I wonder if they'll add a new stretch goal), but giving to charities is all about free will and doing what you want. If you're giving only to receive something, then it's more like a transaction, a purchase. And honestly, most people who are are going to give will give more than the $10 it costs to buy this map. In fact, the most common donor level is 50 USD. All the other rewards are part of the SCAG, which is 40 USD. Combined, you might have just bought the book and the map and had everything you'd get from the rewards of the charity. But giving isn't about getting the rewards; those are just ways of making the milestones in the charity fund more fun for the people at home ("come on people, if we can get it donated, we'll all get to see stuff in this book a few weeks earlier!"). If you're going to give, great. But don't guilt the people who aren't/can't. Maybe consider giving more, since you know someone isn't and you may have the ability to do so that they don't.
But yeah, the charity is good and important. I just disagree with the guy who said, "you're doing it wrong" since that felt less like an encouragement and more like a sneer. Doesn't justify counter sneers, though.