Loads of people kiss my DD, she has always been sociable and friendly, she draws people in. They seem to do it before they've even realised it. I don't mind, it's not an issue BUT none of them have ever done it on the lips, and I think I'd react the same way as you, a polite smile and say nothing, but inside I'd be thinking 'wtf'
Most give a little kiss on the head, or cheek, which as I said, I don't mind, I think it's sweet, but on the lips is odd. I wouldn't kiss another persons child on the lips.
I was holding a baby at a mother and baby group a few months ago, I know the mum a bit, but I only see her at the group, as I was holding this bub and chatting away I found myself kissing her on the cheek, I didn't even think about the fact she wasn't my child. I apologised to her mum, but thankfully she is like me and thought it was sweet.
It's like a second nature for me...
My doctor kissed all my babies, snuggled them, made huge fusses over them etc which a friend of mine thought was weird, it never crossed my mind to think so, I've had the same doctor since I was about 2, he's always loved children, especially babies. I think again, it's second nature for him. He was delighted when I walked in one day and plonked DD down in his arms at our 6 week check up, 'You know me so well' he said 