How many people do you think actually take the time to donate if they can get something for free? How can the developers find time to make enough money to live and support this complex component?
Unfortunately, the percentage of people who use open source software and donate to support the continued development of the software is very small. Because of this, open source developers need to be creative so that they can devote more time to working on the open source projects they have started. If more people were donating, I doubt that there would be a doc subscriber program at all.
As a Doc subscriber you also get some great documentation, which is very helpful if you are setting up Community Builder for the first time. Of course, I'd rather have everything for free, but I completely support the CB team in their decision to have a subscriber program.