I am not sure, but I believe that this is all related to their debit/credit card offering, and requires that you take (what I'll term "taxable possession") of the funds, and from there, IB is doing nothing more than your bank would, and would/could send it anywhere you wished.
In that way, they satisfy the taxing authorities (IRS/StateRev.Authorities in the U.S.), and can dip a toe into "cardholder services" for which they already have had the organizational infrastructure in place for many years.
As long as there is a break between trading funds and (what I'll term) "funds designated for outgoing [card] transit", it doesn't phase me.
But I *don't* know details -- only what I'd expect/need to see. It came out last summer, and I wasn't really tempted. But I'm getting increasing grief from my local bank, and, well, ....... It would simplify my life. Hmmmm.