All businesses, large and small, make decisions on who to buy their products from based on sometimes arbitrary criteria such as convenience, personality of the jobber, advertising, etc. In a sense, businesses discriminate when dealing with other businesses.
But, since the enactment of the Civil Rights act of 1964, it is illegal to discriminate when selling goods and services to the public. You can pick your suppliers but not your customers.
Carried to its logical conclusion, which it could be, the law would prohibit exclusive purchasing channels in certain specialty goods and services. (These are probably intended to reduce outside competition more than anything else, of course, with no implication of racial, age or gender discrimination. Credit rating, net worth, sales volume, number of stores or employees would be factors, though, and not everyone qualifies.)
The freedom to contract, once thought inviolate, has been eroded bit by bit over the last century. Government permission is required to do business, and once granted it must be done only in accord with ever-changing laws and regulations.
It's all for our safety and well-being, of course.
‘elf and safety, like they say in unjolly old England.