BA and AS have had an interline agreement since the 1970's so I don't understand the whole issue about ticketing or bags. It goes without saying that if one chooses to purchase separate tickets, BA will not check across them. But, that is true even across separate tickets.
The US requires all onwards passengers clearing CBP in the US to be processed through a security checkpoint. There are items, firearms and liquids exceeding 3 oz. being two of them, which may be transported in checked, but not carry-on baggage. OP having had access to his bag, needed to be rescreened.
He also chose a short connection. While there is nothing wrong with it, he chose it.