The outward rationale is that it 'helps domestic production and creates jobs'. Never mind that at best case its neutral on both, while making products and svcs more expensive for consumers and likely resulting in a trade war in an unrelated industry. But likely his real rationale is to deal a blow to alternatives and give the fossils a leg up.
From an environmentally friendly standpoint, its of course the right thing to continue to drive alternatives. But the purely economic reason...is that it keeps American innovation focused on a whole technology frontier. It gives us a sandbox to play in. And it is US innovation that is a cornerstone of what keeps us at the forefront of global competition.