In the US that is more commonly a discount rather than a rebate. In addition, there are companies which purchase bulk tickets or participate in pools which purchase bulk tickets.
Unless the traveler is involved in negotiating the terms of his company's travel, he isn't going to know what tickets really cost.
But, even if a specific ticket is more expensive, it is likely that overall the company is saving through the use of a corporate TA, booking software and the overlay which Concur provides.
There are tons of these threads on FT and they all come back to the same thing.