Hector,
You're quite right, to a degree. The point is if you explain the issue to a customer and explain the risks and have them sign for it, you are covered for professional negligence. It's only negligence when you proceeded and the carpet shrunk.
Plus, if there was a problem it is highly unlikely to go to court because the customer knows perfectly well the issue and that they authorised the job to proceed, despite the issue. It is no different to pre-qualifying a customer to any other issue, stains not coming out etc.
You could of course just LM it!
Simon