They're indeed giving away Docker for free, but part of the reason I hypothetically want Docker in the first place is that I trust the people distributing it to not do anything underhanded with their flagship tool's position as a product which runs thousands of other businesses' infrastructure.

There is a certain degree of trust involved in using a product like Docker since it is so critical to a businesses' operations, and I think a lot of people feel like using any kind of tracking (like mandatory registration to download) erodes that trust - we're forced to sit here and wonder what other restrictions might come in the future, or what other information they might start requiring for use of their product... and uncertainty never pairs well with infrastructure tooling that tends to be very important and very longlived within an organization.