You are using a broad term "reimbursement".  However, the only thing Trump wants to stop is reimbursement for out of pocket copays.  The premiums are still reimbursed under federal law (ACA).  What's not to be reimbursed are copays, sometimes as little as a few bucks, that Obama started reimbursing by executive order.  It was never approved by Congress.  That is why Trump can counter with his own executive order.  You got to realize that Obama overreached and abused his power by circumventing Congress on many occasions via his executive orders.  It's Congress that has power over the federal purse, not the President.