>1 - https://en.wikipedia.org/wiki/Robinson–Patman_Act

I'm not a lawyer, but a cursory search shows there's a bunch of carveouts. It's not a straightforward ban on all favorable pricing for big players. For instance, if there's an actual cost justification (eg. bigger buyer = more efficient shipping) that's allowed. Same with offering volume discounts that are available to all buyers. That's not to say everything's above board, but it's not as simple as "wow big box stores get better pricing than mom and pop shops, so there must be federal laws being violated".