Yes if that inability is caused by GDAX, whether intentionally or not, thereby preventing users from being able to manage their positions. There are several ways to interpret their terms, and that is only one of several problems with how they choose to act. While immoral, unethical, possibly illegal, the margin call itself is within their right. Without going into terms again, based on their terms, section 3 and down, they should not have accepted the order as it was an obvious manipulation. Access to the system was down for well over 5 minutes, and trading was halted. At that point only cancel orders should have been possible, not the opening of new margin call orders.