i don't think it's a genuine argument. i think it's a cover to explain why they're not obliterating their own investments. but, let's work this through, anyways.
so, the claim is that bombing the oil will turn the villagers against the anti-isis coalition and, therefore, we should not bomb the oil. the ramifications of this are quite profound. do the villagers feel aligned with the ideology of isis? is it some kind of stockholm syndrome?
it goes beyond pragmatism. pragmatism would suggest that you co-operate with whomever has the ability to get you the oil, and switch sides as soon as possible. the concern that it would radicalize is a pretty devastating attack on the mindsets of the locals.
i would like to hope that bombing the oil would help the locals revolt against isis. and, if it will not, then i'm sorry: these civilian populations are valid targets.
as mentioned, i think it's propaganda. but, insofar as there is some truth to it, it is an argument in favour of civilian bombing - and not an argument against bombing the oil wells.
logic can be harsh sometimes. it doesn't always respect civil rights.