So, even if the people of Easter Island knew they had a major problem on their hands (which, actually, I'll grant you), we don't know if they'd have really identified the same thing we do as the cause, or the same thing we would as the best way to fix the problem. They probably didn't understand how the system worked, to be able to manipulate it reliably.
If they farmed (and my bet is they did - or at least performed basic horticulture), than they know that plants need to be watered, have seeds, etc. I get your basic assertion - that we cannot look back on past cultures using our base of knowledge - and I fully agree. But at the same, we can't look back on past cultures and assume they were working from a blank canvas.
Agriculture (and thus, the knowledge of how plants work) goes back to the stone age, and we can safely assume that these Easter Islanders knew the basics. They probably didn't know WHY seeds worked the way they did, but they did know that plants begat other plants, etc. And yet, they cut down that last tree because their priorities were skewed towards competition with other local groups, and because the priesthood thought it was a higher priority.
No matter how you look at it - even in the context of their perspective towards their own religious beliefs - this was a very stupid thing to do. And I imagine a good chunk of the population was saying as much - but those in power felt it was their only choice. And this... is dumb.
To give another example where I *don't* fault the culture would be the mesopotamians. These guys had farming techniques that were not sustainable, due to soil salination and decline. There are ways to get around it, but they didn't know that, and so when their fields produced less and less food, they turned towards their priestly caste for guidance. This, from their perspective, makes perfect sense, even though it's ridiculous by modern standards.
But to see your means of subsistence destroyed to erect massive stone "gods", to the point where your subsistence is entirely removed, this is something they WERE aware of. And they did it anyway.
And I think that says something about human nature, because we're still doing it today. And if you think about it, that's a very scary thought.