My understanding (IANAL) is that that is illegal, and those retailers should be prosecuted as well. That is essentially price fixing, because the retailer is enforcing their competitors have the same price, using the supplier as an intermediary.
> many manufacturers have similar requirements
That is a different situation, and is AFAICT legal in the US (but not in many other countries, and IMHO probably shouldn't be legal), at least in some situations, but there are limits.