This is one of those things that pissed me off, and I could never quite figure out why, like people demanding hot water instead of cold, it's a trifling inconvenience, and then it dawned on me. You bring someone a water - for free - and they immediately begin to bargain with you. They want the water for free, now they want a lemon for free. If you let them they'd demand their meals for free (and some have). It's just fucking bad manners. If I owned a restaurant I'd throw it on the menu - "Add a lemon to your water - only $5.00", an upsell for everyone that didn't know to leave well enough alone...
a miscellany of completely unrelated thoughts...