Is it legal in the US to open for example a restaurant that can only accept Bitcoin as payment?

My uncle has a bakery and he initiated this question what if we accept both cash and crypto? Then this question came into my mind what if we "only" accept Bitcoin?

I remember a long time ago a person told me as a fact that businesses in the US can not deny cash/card payments but I might misunderstand.

I am sure someone has asked this type of question before but I just wanted to also know more details about which law exactly will be violated if such a thing is illegal.

Also would a restaurant (physical entity) and an "online business" make any difference?