They won't explain, well they do but its the scripted party line of security they are hiding behind.
I did however get a call back from someone supposedly higher up that agreed that the questions being asked are killing 2 birds with one stone, whilst being (cough cough) flexible and allowing us to opt out of the secure invoicing after giving them all the information they have no right to obtain from us in the first place!!. Turnover.. seriously, how does that relate to security in anyway.. And bank details.. they already have that.