Why?

I backed it early and paid less for a half-implemented editor but wanted to show support.

The later you back the more that's actually built.

What main motivation do you think the seller has? It's basically an early bird discount.

Many things improve but go down in price. Can't articulate fully but it instinctively feels like a rationalisation rather than a reason.

>It's basically an early bird discount.

When has that not been a marketing tactic

Exactly. It is a marketing tactic. A very fair and obvious one, at that. So what? Hardworking programmers are not allowed to market their stuff?

I don't see the issue. Price of a company's stock goes up as the company continues to become more useful over time and no one thinks this is unreasonable. Why not same for a product?

Was the same licensing method for Minecraft in beta

And is the same model used by many 'early access' titles, pay less up front for the incomplete game to fund the completion of the future full-price game.

Interestingly, I've seen some devs turn the concept on its head and charge more up front, kind of to say "only buy in now if you are really doing this to support development, and in return you'll get an early release build - but you aren't buying the game early."