There is a surprising number of people who use the words "jail" and "prison" interchangeably

Because they can be used interchangeably.

To repost a comment I made to someone else:

Dictionary result for prison

/ˈprizən/Submit

noun

1. a building in which people are legally held as a punishment for a crime they have committed or while awaiting trial.

And

Dictionary result for jail

/jāl/Submit

noun

1. a place for the confinement of people accused or convicted of a crime.

These are the definitions you get for each word when you look them up.

There is nothing wrong with using them interchangeably.

Languages grow and evolve. The two words are interchangeable now.