If the Left's platform was explicitly "We are an anti-imperialist check on Western power" then your argument would hold. But the actual platform is usually "Women's Rights and Gay Rights are Universal Human Rights"
You cannot claim such universal moral authority when it's convenient ("Justice everywhere!") and then retreat to the safety of a particularist stance ("I only care about what my tax dollars fund!") when it's difficult.
If your "basic human decency" only activates when the aggressor is your own government, then you aren't actually standing in solidarity with the oppressed. You are implicitly arguing that a victim's value depends entirely on the nationality of their killer.
Focusing on more tractable causes makes sense (fwiw I think there are more tractable causes than both of those geopolitical conflicts: millions and millions die annually from preventable global health ailments)
Not unless you're a cynical, murderous psychopath.
It's an expression of basic human decency.