I truly don't get these kind of patterns, because to me, it just looks like they'll miss out on orders from people (like you), who thought they would get a discount, but then end up not getting it and cancelling their order.

Surely it would make more sense to advertise that customers can get a $5 discount, if they order for $30 or more. That way, if your order only adds up to $25, I would be inclined to order a side that gets me to $30 or more.