Sure, couples will kiss in public, but the difference we are talking about is having two people with their tongues down each others throat. I mean, I've never been where you live, but I would be surprised if it was not considered slightly taboo to be sucking face two feet away from the next passenger on the train.
Well sucking faces on a train would not be accepted, but that's a bit of an extreme example. What I should say is that where I live "french kissing" for a medium period of time on the street is accepted.
This reminds me of how there isn't any topless sunbathing in the United States. It's another thing that doesn't cause a bat of an eyelid in a lot of European countries but is considered vulgar (illegal?) in the U.S.