Last year the State Department completed a review of the Keystone pipeline proposal which made the dubious claim that the pipeline would have low environmental impact. (NY Times link) The basis for their argument was not that tar sands oil extraction is ecologically sound but that that oil is going to be extracted from the tar sands regardless of whether the pipeline exists or not. That strikes me as an incredibly tortured interpretation of what it means to do an environmental impact review. As an ethical justification this is like arguing that it's okay to drive the getaway car from a bank robbery because "hey, those bank robbers are going to rob that bank regardless of who's driving them away." Or try that as a response to a question of whether your participation in the getaway drive contributed negatively to the overall crime rate in the city. If your doing X facilitates the performance of P and that is the intention of X, then surely any rational agent concurs that assessing the impact of X must also involve assessing the impact of P.
In fact, this isn't just an abstract moral argument, but I think there's a very clear practical reason to worry that this very argument creates an incentive to extract oil that might not otherwise have been extracted.
Consider that the review is becoming relevant again because the the review writers acknowledged that there is a scenario in which their denial of impact foundered. If the price of oil is so low that it's not economical to extract unless there's a pipeline to transport it away, then even the State Department's denial of environmental impact falls apart. In that case the willingness to supply a pipeline becomes a prerequisite to the continued extraction and the refusal to create the pipeline has a hugely positive environmental impact. So, in that case, arguably the current state, not even the State Department can argue that Keystone has no environmental impact. It does, it makes the tar sands extraction economically viable and without it, they'd stop pumping.
But, let's consider the original argument again, i.e., that if we have reason to think the oil production would continue with or without Keystone then we can ignore the effects of oil production when assessing the impact of Keystone. Consider what effect that has. It provides an incentive for oil producers to continue extracting oil from the tar sands regardless of the price. If they do that, then even in a low price scenario the State Department can invoke the original argument and say that oil production will occur regardless, so now we're again justified in arguing that the Keystone has minimal environmental impact. So, in effect, the State Department has removed an important economic disincentive to tar sands extraction. The State Department's report itself with its specious moral reasoning may well have or may have had a negative environmental impact by creating an incentive to continue pumping oil.