Happened to me many times but just as you, they're not forcing me to do it as it requires some verification of where the funds came. I didn't fill out any of what they're asking for including the IDs because my thought is that I've already provided them some identifications and what's more they need about it? Funds can come from everywhere and it's easy to say that it came from my long term holding and I happen to sell it in some exchanges and transferred it to them for another trade.