So they could shut it down with no need to notify the city like NBH appears to need to do with Westminster City Council. (Even thought the right of way is BBC property).
It will be possible to walk through the foyer, primarily people getting off the train and heading for the Principality Stadium was how I heard it mentioned by a BBC Wales grown up. I'm not sure why they'd want that, but hey. Like any other private building they can lawfully shut the front door to anyone they like (a bit like anyone's home etc)
Still don’t know why they’d take the security risk in opening it up. I realize I’m thinking in US terms with regular (just the kind via email), bomb threats and then risk of shooters targeting media organizations.
I'm sure the BBC will control access according to the UK authorities' security level status. At the end of the day it's a moderate interest building, in a provincial UK city, no worse or better than the large shopping mall a short distance away.
'Shooters' as you call them are way down the list of potential nutters in the UK