I'm thinking more of a safety concern, though maybe they could make it less of a problem than Avril Lavigne was by making sure there isn't a massive load of crowd movement just before. So nothing silly like Chappel Roan before Neil Young for example, as the entire crowd would move across.
Me too, I absolutely love Gaudi's work, and have spent lots of time photographing it (along with thousands of other tourists) and those shots obviously include other tourists at times.
I don't think they try and police it, but it's there, (apparently) and could be used to protect people from behaviour like this. Could this be where the UK is heading?