I generally agree with you, but to be fair to the complainers, what sucks is that Docker didn't make it clear up front that it should be part of your accounting. I don't know if they always intended to monetize this way (if so, we'd call that a bait and switch) or if they sincerely had other plans that just didn't pan out, but either way the problem is the same: There's a trend across all of technology of giving your stuff away for free until you become the obvious choice for everything, then suddenly altering the deal and raising prices.
That kind of behavior has in the past been deemed anticompetitive and outlawed, because it prevents fair competition between solutions on their merits and turns it into a competition for who has the deepest war chest to spend on customer-acquisition-by-free-stuff.