I think it is because merchants set such a rule out of consideration for their own profits that commercial places that provide food are not allowed to bring their own drinks, because many times, merchants rely on products such as drinks to drive their own profits, but if a restaurant wins by dishes, it will not set such a rule, because for them, the operating profit is enough, and there is no need to limit the profit of drinks.

I once met such a hotel when I was working part-time. Because this hotel just opened its doors at that time, I was not particularly familiar with some regulations, so I blindly followed other hotels to set up this slogan, which did increase a large part of the turnover at that time.