So, they receive an order that has a Previous Customer discount code. They do an internal audit and discover that as a non previous customer, you don't qualify for the Previous Customer discount and they cancel your order.
Could they have asked you if you want to move forward with the transaction at the non discount price? Sure.
But maybe, after talking to you, they have decided that you aren't a customer that they want to deal with.
Frankly, I don't have a problem with this.