I've been really frustrated with the common 3 day wait so I looked it up in the SEC regs. (you know, it's all online now) What I found out was that its up to broker discretion wheather they require settlement. The SEC says the broker may extend the buying power prior to settlement if "it is reasonable to assume they will be repaid". Because they control just about everything, it seems certain to me that they will receive the funds on settlement.
Up until several years ago, it was common for the brokers not to require settlement on cash accounts before replentishing buying power, but then there was another Mob cry out against people daytrading their IRA accounts, and they all tucked their tails and required settlement.
I admire IB for showing some spine and ending the settlement requiremt.
I'm sure fear of the SEC is not the driving force, but some broker self interest like making cash accounts so unattractive that any normal account would want to be a margin account where the brokerage actually owns the stock (subject to their pledge to you) and loan it out and make interest on the margin whenever possible.
As such, almost all of the direct access brokerages are not very interested in IRA (cash) accounts.