They were probably offended because they realised that they were behaving badly. People *hate* admitting they're in the wrong.
However, I don't think any reasonable request when framed in a polite way should cause offence. You were trying to do something perfectly sensible, typing in your pin in private. The same would go for asking someone to move so you can get your pushchair down the aisle (or someone asking you to do the same), asking someone to pass you a 'next customer please' sign or asking someone if they could move their shopping so you can get to the empty carrier bags underneath. People are bad about all these things because they are thoughtless.
no subject
However, I don't think any reasonable request when framed in a polite way should cause offence. You were trying to do something perfectly sensible, typing in your pin in private. The same would go for asking someone to move so you can get your pushchair down the aisle (or someone asking you to do the same), asking someone to pass you a 'next customer please' sign or asking someone if they could move their shopping so you can get to the empty carrier bags underneath. People are bad about all these things because they are thoughtless.