On the Fact that Radical Islam Is a Much Bigger Threat to Russia than it Is to America (the Fact that it's Closer in Proximity to Muslim Countries, Has a Much Bigger Islamic Demographic, etc.) and, so, Yeah, Maybe it Isn't the Dumbest Idea on the Planet to Cooperate with Them at Least on this One Issue

Plus we worked with Stalin who was a bazillion times worse to get rid of Hitler and, so, yeah, there is in fact some precedence here as well.