I've noted this practice in many other jurisdictions.
The contributions are coming from a union, not the individual officers who are accused. So I'm not sure this is necessarily a classic conflict.
As a general matter, decisions not to charge aren't judicially reviewable, and DAs rarely explain them.
It's that what we probably care about the most isn't the money, it's the power. Were these defensible decisions or not. We simply do not know.
And DA Clark should take pains to explain them.