Hardly had the ink dried on its written promise not to grant any rebates when it began granting them to the Standard Oil Company.
In those days the railroad rate was not the sacred, immutable thing which it subsequently became, although the argument for equal treatment of shippers existed theoretically just as strongly forty years ago as it does today.

The rebate was just as illegal then as it is at present; there was no precise statute, it is true, which made it unlawful until the Interstate Commerce Act was passed in 1887; but the common law had always prohibited such discriminations.

In the seventies and eighties, however, railroad men like Cornelius Vanderbilt and Thomas A.Scott were less interested in legal formalities than in getting freight.

They regarded transportation as a commodity to be bought and sold, like so much sugar or wheat or coal, and they believed that the ordinary principles which regulated private bargaining should also regulate the sale of the article in which they dealt.

According to this reasoning, which was utterly false and iniquitous, but generally prevalent at the time, the man who shipped the largest quantities of oil should get the lowest rate.
The purchase of the Cleveland refineries made the Standard Oil group the largest shippers and therefore they obtained the most advantageous terms for transporting their product.