What exactly are the national security implications of using Russian engines? That it will combine and burn fuel and oxidizer like a stinking, dirty commie?
It's not wise to trust Russian-built anything, especially with diplomatic relations in their current state. Just like how foreign intelligence services shouldn't trust American-built computers.
What could go wrong? I'm not sure. But why make your attack surface larger than it has to be?
Or Russia could abruptly decide not to sell us any more. Relying on a foreign government to keep our military functioning is not ideal and our relations with Russia, while generally friendly, are somewhat strained the last few years.
The ban is for a specific engine, the RD-180. It's a response to the action Russia took in Crimea, so it's political and they're not hiding that fact. It's related to the national security the same way that waving the flag is. Not at all. Or completely. Depending on your viewpoint.
It's a question of supply reliability. You don't want to depend on a system which may or may not be available depending on things beyond your control. That's true of any supplier, foreign or domestic.
It's not wise to trust Russian-built anything, especially with diplomatic relations in their current state. Just like how foreign intelligence services shouldn't trust American-built computers.
What could go wrong? I'm not sure. But why make your attack surface larger than it has to be?