because "como" on its own is a question word/comparison word (meaning "how" and "like")... and "como" when it refers to "eating" is a conjugation of the verb "comer" which becomes "eu como" when it means "I eat". It could be a little confusing that they are the same word, but you just have to look at the context of the word to see which one it means!
'Like' here is the preposition meaning 'similar to'; it is not the verb 'to like'. So 'one apple IS not like one orange' means that the apple and the orange are not similar. If it was 'one apple DOES not like one orange', it would mean something like: 'the apple is not friends with the orange'. I hope I didn't misunderstand your question.