Apart from understanding the cultural side of this behaviour, you also have to consider the acceptance level of the parent. Some days I would let go, some days not at all, because i'm in a hurry, because it's the 10th time in an hour etc...
Also, someone asking gently to take a picture is ok, someone grabbing my son and pulling him away from me without even looking at me is NO.
If we go to the beach and one lady comes to take a picture, it's ok, having 10 people around us is NO !
If we go to a quiet beach to go away from the crowd and have a guy taking 20 shots of my son who is playing naked in the sand, it's a NO NO and NO !
If we are in a plane and someone wants to hold my baby (after asking), it's ok, touching my son's head while he is sleeping and waking him up, it's NO !!
It's not only us who have to accept these differences and gently smile at whoever wants to take a picture or grab our kids, they also have to consider OUR acceptance level, many people take pictures without even giving a look at the parents, so there is nothing wrong with refusing.