At the current exchange rate, they would be losing ~$1400 USD to 'clean' $700. Nobody launders money at such a drastic loss.
Given that they say their minimum 'deposit' is 1 BTC, even finding a handful of extremely gullible marks will be quite lucrative for them. That's all the motive necessary. It goes without saying that no one who sends BTC to this address is going to see anything back.