I was "unfortunate enough" to learn this technique while working in a commercial with a beautiful starlet. The actors are placed at an angle that prevents the camera from actually "seeing" the actual kiss. The actors make the head movements of kissing, but the lips aren't actually touching. This is commonly referred to as a "Hollywood Kiss".
(addition)
Not to say the above answer is wrong, for there are certainly times when this technique is utilized, but there are obviously plenty of times when the viewer/audience has full visibility of the kiss, and in those cases, it's as simple as professionalism. The actor/actress is professional and has to perform said act making it convincing, but... no tongue, unless the part calls for it.
..................... wat.
C'mon kid, are you screwin' with me? If you aren't, well... Wow... Just Wow.
They do it to ACT out either an act of affection or sexual-conquest/forced or whatnot.
It's not like the actors that kiss in movies go on backstage after the take and start Frenching or making quick, hot monkey-love behind the trailers on their lunch break.
It's just pretendy-time. 'kiddo'.
+50 to your current Emotional, Mental, and Physical Virginity.
Die in a fire.
~ William Kage (Epsilon: In Honor of New Lyrae)
The cast of KIss - 2009 includes: Cassie Laver Nick Rapson
The cast of The Kiss - 1985 includes: Omar Ebrahim Anne Pigalle
The cast of Kiss Off - 1998 includes: Marcie Saunders as Jill
The cast of A Perfect Kiss - 2011 includes: Kelli Klymenko as himself
The cast of Kiss - 2013 includes: Peter Nicholson as Husband Lisa Shelley as Wife
If you see the actors using their lips to kiss any part of the other actors body, technically they are "truly kissing" but it is part of their job if it requires the actors to kiss in the script.
The cast of How to Kiss - 1988 includes: Chris Hoffman as Narrator
They are actors in the 'Harry Potter' movies and their characters kiss in the movie.
The cast of Kiss Me - 2001 includes: Guido Preger Ernesto Simoniello
The cast of Angel Kiss - 2004 includes: Maria Hirai
The cast of A Kiss in the Dark - 1919 includes: Adolf Philipp
The cast of Kiss - 1986 includes: Shigeru Muroi Yoshikatsu Ohshita
The cast of Kiss of Medusa - 1982 includes: James Andriani
The cast of Kiss Vakond - 1994 includes: Lea Tolnai
The cast of Elektrik Kiss - 2010 includes: Falcon Stein
The cast of The Kiss - 1985 includes: Omar Ebrahim Anne Pigalle
The cast of KIss - 2009 includes: Cassie Laver Nick Rapson