This is not about the PO staff asking what is inside your parcel - I do think that's a fair exchange for them carrying your stuff to it's destination.
It's letters. Perhaps it's just the PO that I go to but today, I was returning an out-of-date passport for renewal. The Passport Office say you can return it by standard post or special delivery. The Post Office wrongly claimed that the Passport Office recommend special delivery which was more expensive than standard first class by a fiver!
I said no, I'll send it first class it is out of date anyway. I was then told it was still personal data and the loss wasn't covered by first class mail and when I still requested that it go first class, they wrote on my receipt and said that was to indicate that they had told me about the data risk and I had declined

I wouldn't mind if they were right, but they are wrong about what the Passport Office recommends! I need to find another PO which isn't such a hassle to use, I just want to send an envelope without a written note afterwards!