Earlier quoted context omitted.
That reminds of the sign "Buy one, get one". Does that mean you pay for an item and you get that item, or you pay for an item and you get another item for free? I expect the meaning to the be the second one, but my brain parses it to be the first one.
That’s because it they have left off the word “free” - it’s just an odd shortening of the less ambiguous “buy one get one free”, probably made by someone who is very familiar with seeing “buy one get one free”
It’s also used more generally for buy one get one X% off sales and you’ll see lots of “BOGO50” promo/coupon codes. Then marketing jumped on the bandwagon and started plastering “BOGO” all over marketing material so consumers are used to the lingo now