They’re worried that if there was an incident they might get sued for having inadequate security.
I’ve spent a lot of time going to places with security like this (airports, museums etc) and getting a belt with plastic buckles (you can get them at outdoor clothing stores) saves a lot of hassle.
"Certain items cannot be brought into The View from The Shard including large bags and backpacks, over the size of 22 x 15 x 8 inches (55 x 38 x 20 cm) or other large equipment."
"We ask guests to walk through metal detectors, which use a low-frequency electromagnetic field to look for metal items."
https://www.theviewfromtheshard.com/frequently-asked-questio...
Next door you need to go through a scanner to get up to the restaurants on the 31st and 32nd floor of the Shard.
Security in these buildings is very arbitrary. I can get that some of the more high profile places feel more of a need for security, and assume that anyone up to no good will perhaps not be the brightest of the bunch and/or will go for more symbolic targets, but still.
Incentives align among all three.