So, you can only place an order that, at the time of order entry, you intend to get filled. However, as the market changes, your intentions may change, and that is completely fair. So, while you may initially want to get filled, if you see the market moving against you, you may decide you want a better price, and then move it or cancel it.

So, it's all about initial intent, which is extremely difficult to prove. And that's why most traders who are well known to spoof markets haven't been prosecuted (see 3Red's Igor Oystacher), because they are smart enough to hide their intentions.

Sarao, on the other hand, wrote emails where he stated exactly what he was doing. For example, in the indictment, they have emails where he wrote that wants an order type that will never get filled, and if it looks like he is close to getting filled, to immediately cancel. Without those emails, he would've been able to at least make up some defense. When he has stated in writing that he is placing orders that he does not want to get filled, well, that's a very prosecution.