Why would this be required by the US government? And why the only broker out there asking it's clients about this is IB and no other broker is asking for it (If it's a US government request?)
For the case of a US-citizen, why the US government is asking IB to get the SOW of it's clients, why aren't they doing this through the IRS instead, which I believe they already have this information.
I don't really believe this is a US government request. IB compliance wants to know the source of wealth of their customers in anticipation that when those customers wire-funds to their accounts in the future, they can immediately identify if those funds are legitimate/clean or not (Based on the SOW documents they submitted earlier) and consequently they can detect suspicious cases/funds at a very early stage.