This is a bit like asking, "Are cats ethical?"
Cats are just cats, and the oil sands are, well, sand with oil in it. Ethics, which relates to certain judgments we make about human actions, does not inhere in the physical world.
Were we to ask a different question - "Is industrial exploitation of the Athabasca oil sands a good idea, given the social and environmental costs involved?" - the answer might well be no.
So what's gained by phrasing it in ethical terms?
It's a bit like describing every accidental death of a child as a "tragedy," meaning something not only sad and untimely, but sad in a grand and universal way. Tragedy, correctly used, refers both to a particular type of dramatic plot invented by the Greeks, and to a philosophical belief they had concerning the connections between personal will and destiny. Tragedy does not mean just really, really sad.
Ethics, too, has a specific meaning, though there's less agreement philosophically about what that is. One thing that's clear is that it refers to human choices. The act of squeezing oil out of a bunch of sand can't be either moral or immoral. The choice whether to do so in the face of grave concerns about the effects on the local environment, on first nations, and on the global ecosystem can, indeed, be a moral choice.
But even here, there's reason for doubt. Why don't we content ourselves with debating whether certain projects are smart or dumb? Surely if I propose to lick a doorknob when the temperature is minus 30, we can all agree this is a really bad idea without calling it unethical.
It's as if we've lost the public confidence to weigh the sensibleness of actions without calling in the heavy artillery of big ideas. The trouble with big ideas is, they're generally absolute. We can argue the degree to which developing the oil sands is smart or dumb and hope to reach some consensus, but, once we bring ethics into it, we can only entrench ourselves in opposing beliefs.
That's a loss for us all, just as calling every accidental death a tragedy risks diminishing our sensitivity to genuine human sorrow.