Companies are required to follow the law.

These laws do not prohibit putting up political banners, but Amazon certainly cannot do whatever they want.

There are laws regarding price fixing, abuse of monopoly powers, discrimination on a protected class, product labeling, and making false and misleading statements about drugs.

If they sell Cuban-made cigars made with conventionally grown tobacco, then while they technically can put up a banner claiming "these organic, made in the USA cigars, if smoked twice daily, will cure epilepsy in children - buy now!", they'll have broken several laws.