HR was called "Personnel" in those days. It was different.

The reality here is that an intern doesn't have civil service protection, so it is quick and easy to dispose of them. Going after the supervisor may take longer, so if rapid action is needed, they'll fire the first person who serves "at the pleasure of" the executive.

Oh, I don't think people are any worse - I just think there's more adherence to the letter of whatever regulations there are now. The principles of zero-tolerance (three strikes...) have been widely applied, regardless of the nuance of some situation.

Note also that it turns out the OP was "fired" but immediately rehired too...