I suspect they are confusing warranty with your right to return within 14 days.
If you were to remove the plug and then damage it through incorrect connection, I suspect you'd be goosed.
Making a correct final connection to an appliance via an outlet plate, with supply fused down to the manufacturers specification, should absolutely not void the warranty. I have done so on many occasions and would vigourously contest any withdrawal of warranty cover for doing so.
That said I rarely do it these days, sockets in adjacent units mean that I don't have to go back once appliances pitch up, and fitters can just whack them in and plug away.