You are correct in that Masks are ineffective for protecting the wearer, as the mask is not a fully sealed shield against airborne viruses. What they have belatedly realised is that they are effective in reducing the aerosol spray of viruses from an infected person, as the mask prevents coughs or sneezes from becoming airborne virus droplets. It doesn’t protect the wearer, it protects other people from the wearer. Given that you can be infected without knowing that you are, and that you can be contagious for weeks without having any symptoms yourself, it is a service to your community to wear a mask when you go out in public. This shows consideration for other people, and gives them some confidence that you are not a disease vector.
Compare the deaths per million population for countries where mask wearing is advised or mandatory to those where covidiots protest against their use, and I think you will see some evidence to support their use.
Businesses have a right to reject patrons who do not comply with their dress code. It is known as ROAR: right of admission reserved. In the same way you cannot enter a building site without wearing a hard hat, it is reasonable to refuse entry to a supermarket if you refuse to wear a mask. This has zero in common with rejecting people of the grounds of race or creed.