I tend to agree with Su Ki. May doesn’t necessarily mean “is permitted to.” That’s what English native speakers are taught in school, probably to contrast the word with “can”, “to be able to”, probably because the two words are used so often interchangeable in the spoken word. “Can he come in now? (is he permitted to?) “Yes, he can come in now?” (He is allowed to come in now.) Native speakers are taught the difference in English classes, but trying to impose grammar rules on native speakers is rather silly, and artificial, unless you want to teach a literary style fit for publishing. The way native speakers speak determines the rules. Not the other way around. Native speakers don’t think about grammar rules when speakering. They go by what sounds right. Non-native speakers need grammar rules as a guideline to contruct sentences and communicate in a foreign language. And that’s fine. It’s serves a useful purpose. There’s probably some source in English that officially sets forth the proper use of words in English, but I don’t know what it is. In my second language, German, we have Duden, which is an authority. What Duden does is give a numbering system like 1,2,3,4 showing various meanings of the words and uses, regional variations, whether the word is considered elevated speech style, colloquial or standard. These differences in meaning and context would be clarified in Duden and show that using a word like may, that can have varying meanings based on context, would resolve any confusion and help people get away from inflexible grammar rules. We probably have something similar for English, but I don’t know what it is.