My late father was with AOL/talk talk.
It is three years since his death and I wound up the account but last time I checked, his old aol account was still accessible, his mailbox still populated. I wish it would vanish, but haven't the heart to delete it all.
I suspect the reason they do it is simply the mailbox is of commercial value to the ISP. The fact that my cousin Doreen emailed my father about her Vauxhall Astra is probably of no interest, but much of the mail lying around in old accounts will hold data that has saleable commercial value, eg to advertisers.