Some early access games are highly questionable at their quality i am sure, but the thing is that people are willing to take that risk apparently.
Unless they couldn't read, early access clearly states:
Early Access Game
Get instant access and start playing; get involved with this game as it develops.
Note: This Early Access game may or may not change significantly over the course of development. If you are not excited to play this game in its current state, then you may want to wait until the game progresses further in development.
And then there is a button that says "learn more" which states a lot of things including:
"You should be aware that some teams will be unable to 'finish' their game."
They warn the consumer of the fact that the games can be unfinished at the time of purchase and might stay that way.
It shouldn't be a thing, i agree, i rather would like to see every game in a good playable state on Steam, but apparently people think otherwise and actually buy a lot of the early access games.
This can be reinforced by the fact that there are multiple games on the top seller list that are actually early access.
The consumer should be more careful before they purchase things.
It should be comparable to buying the game on DVD in a store, i can see your point, but on the same point, the consumer is getting warned of the risks and is apparently willign to take these.
Who is really at fault then?
According to outdated laws, Valve is.
Times change, laws are often a bit, if not very, outdated.
Some of the laws in countries cannot even handle online purchases because they didn't have that at the time of writing the laws about purchasing stuff.
I am not completely defending Valve however, but i think that if you cry about buying into an unfinished product even though you have been warned multiple times, you should be a bit ashamed of yourself.
Take the risk and face the possible consequences or just be patient and see if it gets released and if the reviews and such are good.
Same with prepurchasing, a lot of people are crying about their prepurchases being the mistakes of their lives.
I too have done some prepurchases of which i didn't like some, but that is the risk you take.
You are buying it before the reviews and such are even out of the game, so there is no way to guarentee if the game is any good.
You ought to think carefully before you buy a product.
That is how it works in this capitalist world we live in.
TL;DR
Do research before you buy a product or you might find out it is not worth it.
Do not believe every commercial or other way of promotion of the game by every word, sometimes they may mislead you, accidentally or on purpose.
