Question: Why is there an issue of mar’is ayin by a certain case in the laws of Shabbos but not in a certain case by the laws of Kashrus?
A. Mar’is ayin is causing others to think you are doing something wrong when you really aren’t. You can’t put yourself into that situation, for example walking into a McDonald’s to ask for a coke and people might think you are buying non-kosher food.
B. The Rema in the Mishna Berura (244:1) says one cannot hire a non-Jew as a contractor to build a wall around your house if he is going to be also working on Shabbos because people with think he is being paid a daily wage which is certainly forbidden to do on Shabbos rabbinicly. And even if one lives in the middle of nowhere only among non-Jews there is concern that a guest who comes or one of your household members will suspect you of hiring him as a daily wager on Shabbos.
C. A contractor can technically get something done for you on Shabbos because you are not paying him by the hour or day, rather to just get the job done so working on Shabbos is for his own convenience not for your benefit it is just forbidden because of mar’is ayin since people will say he was hired by the hour to work for you even on Shabbos and the rabbis forbade non-Jews to work for or give benefit to a Jew if it will be doing something a Jew cannot do on Shabbos.
D. The Nachalas Tzvi in Shulchan Aruch Yoreh Deah (87:3) said that we are allowed to cook chicken in almond milk because milk and chicken is only rabbinically forbidden and though the Shach and Maharshal argue on the Rema and says there is an issue of maaris ayin even for rabbinic matters so it should apply in this case too however the Rema holds that surely maaris ayin applies in rabbinic cases but only in cases which is for sure maaris ayin but here in this case where chicken is being cooked in almond milk inside the house there is no concern of a prohibition accept for what the rabbis enacted that whenever there is an issue of maaris ayin that’s even privately in the house but that’s only for Torah level mitzvos but here even if he is making it in front of his household it is considered private because they for sure know what he is doing.
Answer: By the Chicken in almond milk they can see what he is doing so there is no room to be mistaken but by the shabbos case not everyone knows the business of the head of the household so there might be what to be suspect.