Tita Girl:

Congressional compensation before the ACA consisted of a salary plus federal healthcare. If they switched to the ACA and were required to pay 100% of the new premiums, that would have meant a substantial reduction in their gross compensation. The subsidy they receive is equivalent to the cost to the government of the federal healthcare that previously had been provided to them. The effect of the subsidy is that their gross compensation is equal to what it was before the switch to the ACA. 

As a business owner or board member, I have had several instances where an employee had reason to request that they be allowed to opt out of the company-provided healthcare in order to purchase their own healthcare. We agreed and paid them what the cost of their coverage had been to the company. That was fair. Their compensation remained the same and there was no additional cost to the company. The Congressional switch from federal employee coverage to the ACA worked the same way.