there is certainly no rule to warm up. it is just a customary thing that people do. you are certainly allowed to just stand there and not receive any shots if you wish to. what your opponent is objecting is that they are used to people playing decent shots while warming up and they tend to use that to gauge opponent. where they failed is that they didn't realized people don't need to play their best during warming and they mistakenly relaxed their mental expectation. what they missed is that they should play as hard as they can regardless of how their opponent look. this is a tournament afterall and not some social game night.