Manufacturers selling direct via ecommerce is a very new phenomenon (last 10 years). Before Amazon, there was an entrenched network of distributors and exclusive contracts in both directions of the supply chain. Amazon has busted this wide open.
It is important to understand that FBA largely pushes the administration and supply chain overhead down to small time individual resellers, utilizing their willingness to do free labor.
Amazon encouraged this behavior, gave blind eye to counterfeiters, and making ecommerce too hard to ignore for mfgs to compete on directly.
IP has never been efficient to enforce, old school exclusive/protective contracts masked this, but now the emperor has no clothes.