Business isn't static. Costs and operations are not static.
At one point Docker probably may have had an authentic intent for a free service, but costs along the way changed the reality of operations and long-term cash flow and success of the business required making changes. Maybe the cash saved from bandwidth is what makes the next project possible that helps them grow the bottom line.
Further what was once a positive value proposition 18 months ago can turn into a losing proposition today, and a company should be allowed to adapt to new circumstances and be allowed to make new decisions without being anchored and held back by historical decisions (unless under contract).
As fun as it is hold executives to unrealistic standards, they're not fortune-tellers that can predict the future and they're making the best possible decisions they can given the constraints they're under. And I don't begrudge them if those decisions are in their own best interest, such as is their responsibility.
I'll give Docker the benefit of the doubt that this wasn't a bait-and-switch, that they never excepted it to become so successful, and that costs outpaced their ability to monetize the success and were eating into cash reserves faster than plan. I think the current outcome isn't so bad, and that we're still getting a considerable amount of value for free. It's unfortunate that some people are only finding out now, and are now under pressure to address an issue they didn't sign up for.