It provides a defense -- you can say 'I wasn't laundering money' if that's their claim for what you did, and you can possibly provide evidence that you weren't laundering money; there's no way for you to say that you weren't violating an unknown term because you don't know what that is.

It’s not up to the companies, who (in my experience) largely hate having their hands tied. Legally, saying something like this can be construed as giving information to the target of the investigation, and doing so can result in fines and even jail time.

It’s dumb, and of course a guilty person will know exactly what happened, and of course hearing a response that their account has been closed and they cannot appeal or get any information as to why is going to make it even more blindingly obvious.