I once booked my bike in for similar, and got a surprise when they fitted a new cassette. I queried it - because the cassette that was on there hadn't done many miles at all, and had been fitted by them in the spring at the same time as a new chain and chainrings after I'd (deliberately, because the chain and cassette were about due and the middle ring in particular was getting rough) run the whole lot on over winter. The chap in the shop (who was the one member of staff there that I didn't particularly like, I have to say - the other staff never patronised me in the way that he did) said it was because of the level of chain wear, they 'always' replace the cassette if the chain has stretched beyond a certain point. I pointed out that I had specifically told them when they assessed it that the cassette and chainrings were only a few hundred miles old, and that I was surprised by the level of chain wear they measured given how few miles I'd done, and that they'd ALWAYS rung me in the past to check before carrying out any significant extra work when they serviced the bike.
In the end they comp'd the cassette. I'd bought the bike there originally, and had bought and had fitted by them several upgrades over the years - new saddle, better pedals, upgrade to BB7s instead of the OEM shimanos, adding the most expensive ergon grips and touring bar ends, a new wheel - on top of routine servicing and consumables.
