It's a men's prison, so maybe it's so the prisoners are less likely to get excited and have behavioural issues later.
ooh ooh ooh I know this one, pick me miss!
It's the first rule of misogyny: women are responsible for men's actions. In this case, women are made responsible for the actions of male prisoners and made to modify their clothing in an attempt to alter the prisoners' actions.
a fair bit of manhandling to check.
Surely the woman, or man, could be asked to lift each boob for a check.
This. A full strip search involves squatting naked in case anything falls out of the vagina, not usually a penetrative search. Asking someone to lift their own breasts is much less humiliating than being asked to squat naked.
In a belvia-type crop top I still fail the pencil test, in fact most of the point of the crop top is to wick sweat away from where it gathers in the inframammary crease. Big tits are a curse.