discrimination isn't illegal, only against specific groups (e.g. race). Discriminating on factors that affect those groups is legal, as long as the intent is not to use it as a proxy for protected group.
You're not wrong, but discrimination in businesses serving the public is a pretty hairy subject.
The protected groups are broad enough and the spirit of the law clearly such that there's a pretty straightforward path to arguing refusing to accept cash at public restaurants/bars is discriminating against minorities which tend to overlap substantially with poverty, or religions that prohibit the use of debt/credit where cash would be perfectly fine.
The fact of the matter is this move towards cashless retail commerce alienates significant numbers of citizens, and those people overlap substantially with the existing protected groups.
If you're going to refuse cash, it should be a private members-only club.
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