Why would you pay good money for a game that could become something completely different than what you thought it would be when you bought it?
If the devs are any good they will take player feedback into account. You know, actually beta the product.
From their standpoint, if people are willing to pay for that, why wouldn't they take advantage?
Once someone has paid you for the alpha access though what incentive is there for you to pay attention to anything they have to say? You have their money and whatever the final game turns out to be that isn't going to change whether they are completely happy with it or not. Go and read some of the early access "from the developer" jargon. A lot of it is "we know not every change we make will make everyone happy but you'll have to trust us." No I don't! Make the game you want to make and then I'll tell you if it was any good with my dollars. That's how the system is supposed to work.
As far as I'm concerned the entire thing is crap and it's totally Valve's fault. None of these developers would be able to do this if the people who controlled Steam had any scruples left at all. But of course they could give a shit so long as they're making their 30% per sale.