That has nothing to do with the suppliers, it is an agreement between the wholesaler and his customers not to sell to the end user.
Example: I run a electronics retail business, so I sell to pretty much everyone. There is an electrical goods wholesaler in town where I buy electrical goods like wires, plugs, sockets, MCB's and so on. We work on the understanding that they (the wholesaler) do not sell to Joe Public who'd like to save a few bob. They sell exclusively to tradespeople and retailers.
They do not even tell Joe prices, they basically kick him out.
If the wholesalers started to sell to everyone all retailers could shut up shop and go home.