They seem to offer much more than just forex. FINRA, SEC, CFTC fined Interactive Brokers last year $38 million for not asking about the source of funds of clients and some other violations.
Nothing shady about it. It's an insane US policy. Not only are they asking about your funds (and proof!) that you use to fund the account but also all your other holdings that have zero to do with US. I'm not an US citizen nor a resident, what business is that of theirs? If any other country did this, there would be issues.