"Inappropriate" feels like too strong of a word for me. "Somewhat tacky" is closer to my feelings. Also, there are a variety of exceptions, such as in areas where you are likely to greet/part someone intimately (ie, airport), or in certain places where such interaction is generally accepted (ie, nightclub).

And it's such a small deal as to be nearly neutral as long as the kissers are stepped enough out of the public way, so nobody has to wait for them, walk around them, or even look at them unless they want to.