Restricted grants are when you give money to a nonprofit to do what you want, rather than what they want. Usually this is some highly visible thing that the donor can then claim credit for. These projects are almost never what the nonprofit would have chosen to do themselves. Often they damage the nonprofit significantly. I was talking to a nonprofit a few months ago that had branches in two different cities quite far apart. I asked them why they'd chosen to expand to another city rather than expand in the city they were in. Wasn't it a distraction? They said it was, but that they'd gotten a restricted grant to open a branch in the new city.
The less powerful nonprofits have no choice but to take such money. And they don't dare complain about it, which is why you've never heard about this issue. Exactly the opposite, in fact: they have to issue press releases lauding the generosity of the donors. Which then perpetuates the problem, by making it seem to future donors that restricted grants are how you're supposed to do charity.
I don't know how Dropbox figured out that they should do unrestricted grants. It's a remarkably sophisticated insight for beginners.