It's been in the news a bit this last few weeks, as at the end of July the National Gallery changed it's policies, to permit photography, as it looks ahead the impact of planned changes to provided free Wi-Fi in its galleries.
More info here.
Here's an old comic which pretty much sums up why they're making the change:
"As the use of Wi-Fi will significantly increase the use of tablets and mobile devices within the Gallery, it will become increasingly difficult for our Gallery Assistants to be able to distinguish between devices being used for engagement with the Collection, or those being used for photography."
"People won't engage properly with the venue and displays, or be respectful in the spaces"
"They're gonna do it anyway, so build it in as a positive feature to enhance the experience."