Question: Why does the logic of zeh vizeh gorem help for if salt was added onto a salted dish on the fire but the logic would not help to allow one to read from a fire that was lit by a non-Jew for a Jew on Shabbos right before his light went out?
A. The Mishna Berura (318:9:73) brings down that there is an argument whether adding salt in a kli rishon off the flame is considered cooking on Shabbos, but definitely adding salt onto a pot of food on the flame is forbidden, either way it is permissible to eat the food because the salt is nullified on top of the food. But this is only because the food that was salted before Shabbos because both the salt that was put on permissibly before Shabbos and the salt that was put into the pot prohibitively on Shabbos, i.e., zeh vizeh gorem, one can eat it.
B. If a person has a candle and because the light is so weak, he can’t read and he transgressed and asked a non-Jew to light a candle for him, it says earlier in the Mishna Berura (276:4:32) that it is forbidden to read from that light.
Answer: Rav Shlomo Zalman Aurbach (Dirshu 81) explains that only in regards to salt, that after salt is put on the food it melts and is not recognizable, the Rabbis permitted eating it because zeh vizeh gorem but by the candles, where the added light is recognizable by all, therefore it forbidden to benefit from it.