Conversationalists will want to preserve ecosystems

No doubt some of them will, but I suspect you meant "conservationists." And yes, I agree that some of those will assign intrinsic value to "nature" in various forms, or at least claim to, as you describe.

Pro-life advocates say that beings without preferences have rights by virtue of future preferences. Not all of them are religious.

Some of them do, yes. Indeed, I suspect the ones who say that are disproportionately non-religious.

why did they decide that those were in fact the deity's preferences?

A fine question.

Doesn't it hint that they might have an underlying feeling of those preferences in themselves, that they would project those wishes on a deity?

That's one possibility, yes.