No they haven't, they've paid for the product they wanted, and did so because they own the program it was designed to run in.

You're pretty much taking the Open Source thing to mean working for free, but again, that is not true. The Blender Foundation have goals that need to be met in order for them to pay the programmers who create much of the program you get for free. This means that there's genuinely no cost to the user of Blender, and the coders who make it happen, unless voluntary, do get paid. With Blender, there are no shareholders to keep happy, so what they do is set goals they wish to meet, and regardless of the time it takes to reach them, do not make a start on them until the money is there to pay for it.

It's the business model of the future, one so open they have a section of their website showing how much money has arrived into the foundation, each and every month, and how long we can expect to wait for projected features to arrive in Blender. So while you are correct that it does get paid for by someone, it is not a requirement of the business model Blender uses, because regardless of whether it got paid for or not, it would still happen eventually purely through volunteers, it would just take much longer, just as it used to do back in the early days.

People are prepared to pay for something that is free because they understand the importance of supporting freedom, and that supporting the Blender Foundation is the polar opposite of supporting something that is closed, can lock you out, abuse you, and effect the viability of your business. No one who relies upon Blender for their business, will ever be let down by it, shut-out of it, or have any part of it they depend upon, taken away.

The fact that you can download Blender and use it, abuse it, sell it, rebrand it, proves that it is truly a free product. You will also never be forced to enter a serial number of have Blender connected to the internet, because again, it would be completely pointless as the product is truly there for your benefit, not theirs.