Hm. On any flight KLM indicates that for privacy reasons you should not take pictures of passengers or crew without their consent. This is very much the same. You reveal to an outsider that person A is on flight KL xyz on date abc and that is information that could or could not be a thing that person A would like to keep private. May be indeed soap opera level (gossip gossip do you know I had your acquaintance on board?) but from a professional point of view this should not happen.

PS Edit, but then we flyertalkers are of course guilty too. We have somewhere a whole thread on revealing if we noted a celebrity on board! I plead guilty since I posted there too ;-)