Sorry, but I have a much more cynical view of why this policy actually exists.
Making public trees available, but NOT downloadable doesn't protect any information. It simply SLOWS DOWN the process of obtaining it. And who would want to slow down the process of obtaining that information?
Perhaps the people who are selling access to it at a rather steep monthly rate?
I understand that when I use someone's tree information, it needs to be researched and verified. But it's drastically more helpful than starting from scratch. I had been hoping, when I signed up, that I'd be able to snip parts of trees and patch them into some of my empty spots to give me a framework for further research. What should have taken hours with the proper mechanics instead takes weeks, and those are weeks when I COULD have been doing the research and verification.