If I try enter a limit order for an options spread I am denied being able to make this trade due to not having a margin account. Why? I'm not borrowing any money to make the trade, it's all in cash. My understanding is that spreads have a limited risk (what I spend on the options) and reward, so why would I have to maintain any margin on them? For example, if I buy an ATM call and sell (write) an OTM call my equity in the former should always be greater than my exposure to the short call, so how could there ever be a margin call?
Even weirder still would be the case where if by some quirk in the market I would happen to get filled on a net credit limit order for a box spread. In this case it is arbitrage that is basically fixing a profit for me at the time of the trade, which my understand is would be risk free. Again, why would I need a margin account for this?