Talking to someone on your phone is prohibited in public transport, as it would disturb those who are around you. While eating in a public place, you are not allowed to speak, unless you are in a social gathering, in which case each group of people are separated by walls for each table. As everyone knows, Japanese people value their privacy over everything else. A prime example of this would be when you visit Japan.
But there's no use forcing people who enjoy doing it wherever and whenever they want as it would only provoke them to go on and make things steamier for everyone to see. It does make sense, given that kissing, cuddling, and even having sex should be done where no one is watching you and your partner. It's nice to be affectionate with your special someone, but doing it in public just triggers ire in those who prefer playful courtship to be done only in private. A lot of people frown upon those who practice PDA or Public Display of Affection.
