The ones who use grape juice also are not ones who are participating in the sacrament but are just play pretend. There are mixed opinions in the Coptic Church on if we should drink or not, some people see it as why take the chance to be tempted to drunkenness and others see it as they have the self control and it is okay. St. Paul's advice is just general advice for anything that might cause someone to sin, if they have a weakness don't be a dick and flaunt it in front of them be mindful of your brothers weakness.