Bishul Akum

Question

Dear Rabbi Abadi, you recently stated Packaged foods sold on the grocery shelf are not subject to Bishul Akum (cooking of a non-Jew). Some of my freinds who learn in a Yeshiva were asking me about this Psak, could you explain why not. Thanks alot PS Their yeshiva is probably one that they dont know how to learn but I still would like to know how to answer them.