You know how you have to have a prescription to get glasses? That's because they're something that a DOCTOR prescribes. I think people forget that when they purchase glasses. There are logical reasons as to why they can't just make one lens and not the other, as the people before me have explained. Any company who WOULD replace just one lens like that is not doing their job to ensure the quality of the product or the health and safety of your vision.
I can certainly understand being irritated at first, but I CAN'T understand why you'd stop giving them your business over it. YOU messed up the lens, and they're willing to replace both of them for half the cost. Think about it this way, dude. You're getting two NEW lenses at the same cost of having one lens replaced at full price (which is what any other business selling anything other than glasses would do).