You realize eventually they plan to have an entirely user-driven store, and completely ditch greenlight? As in, anyone can put anything (that is their own work) up for sale? This is a move towards that. Personally, I'm completely fine with the terms they have provided. It covers their back legally, and its easier than forcing devs to finish their games, or hand out mass refunds.
Emphasis on the last sentence. This is what I do. I get early access because I like what they have so far, and what they promise to do is a bonus. I didnt think Space Engineers would have all the content it would get, but I hoped for it. I bought it because I wanted to crash spaceships together. Same with Prison Architect and Tabletop Sim, and Starbound.
I've been burned by alpha backing as well. I backed Rawbots on their website, and haven't received a single update in over a year, and now their website is down. But I still enjoy the build I have installed, and knew the risks by backing.
I don't think it's valve's job to police it, but they should inform people, which they have done with this. They could do better, sure, maybe a disclaimer before you check out or something, but as the library of Early Access games grows, it'll get harder and harder to decide what is "abandoned."