I told them that I don't charge for the lime, because some costs sufficiently negligible that it's not worth the cost to the brand, or worth the diminished repeat visits to charge. The average Starbucks customer comes in six times per month. Turn that six into a five because you wouldn't give them a lime, and you've just done a horrible financial disservice so your store.
This isn't the first time something like this has come up. The overarching trend among baristas is for them so feel personally taken advantage of when a customer does something that "cheats" the company out of money - like splitting a venti iced drink into two full tall iced cups. These baristas often rationalize their feelings of injustice with short time horizon calculations, without considering that the brand is the most powerful mechanism Starbucks has going for it.
CEOs understand the cost of being penny wise and pound foolish, baristas do not. I doubt this is a coincidence.