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being charged for a warranty frame swap isn't uncommon
It's a funny one... and it's been totally messed up by remote online sales but as I understood it part of the downside to being a dealer of a particular brand is that you suck up doing warranty work. That doesn't stop shops from charging for it, and if the customer has bought from another shop, then they will feel within their rights to do so.
In this case, it's not a frame that has randomly cracked - the design is faulty, it doesn't work as intended. That doesn't seem like a situation where the customer should be out of pocket.
being charged for a warranty frame swap isn't uncommon, a few years ago Sigma quoted me £150 to swap my cracked Supersix for my warranty replacement - it's still workshop labour to the shop that's not covered by the frame manufacturer regardless (obviously i politely declined and did it myself regardless).
As far as i know this is the case for most major retailers like Evans / Cycle Surgery etc too.