I have looked for some grammatical support for your question, but have found none yet. But here is what I think: ‘Non’ is a prefix meaning ‘not.’ Usually a prefix goes in front of a word; and in this case ‘non’ makes the word its opposite. So nonalcoholic would mean not alcoholic, as in nonalcoholic beer. In this phrase, nonalcoholic is also used as an adjective modifying the noun beer. We can go through the same process for non-alcohol, with the exception that this word is already a noun. So, as parts of speech, the two words differ. Additionally, if you are having your discussion based on beer advertisements, remember marketing companies tend to make up their own words.