Q. The law of mar’it ayin states that one should not do something permissible if people will get the wrong impression. Yet Pirkei Avot says, “Judge every person favourably” (Avot 1:6). If I am judging every person favourably, why do we need the law of mar’it ayin?
A. The example you give is of an orthodox Jew drinking water at McDonalds, which might give rise to the impression that the person has eaten a t’refah meal there.
A spectator needs to be helped to judge the situation favourably, and so it is best not to do anything which they might misunderstand. The Talmud says, “Wherever the sages have forbidden a thing for appearance’s sake, one should not do it even in strictest privacy” (Betzah 9a, etc.; cf. Shulchan Aruch 301:45).