Hello. The problem is that فَتَحَ/يَفْتَحُ does not mean to "turn sth. on". It means "to open sth." (anything, like a door, a box, a book, the faucet). But you can't use فَتَحَ/يَفْتَحُ to say e.g. "I turn on the radio." ("to turn on" is a completely different verb). So as this course is about learning Arabic and not about learning English it makes more sense to teach the correct meaning of the Arabic word, even if often it does sound a bit weird to a native :-)