In the UK most restaurants and retail outlets display a sign that say "The management reserve the right to refuse service"
If this sign is displayed and some one is asked to leave they must do so or the police may be called NO reason should be given.
If a reason IS given and that reason is illegal (prejudicial) then it may be contested later in court unless the objection is withdrawn, but if NO reason is given it is perfectly legal for an owner to ask anyone to leave their premises so long as it is not designated a public space.