Hot hatches

I recently purchased a 2022 Renault Megane Trophy from a dealer as a demo/used car that had 9000km on it. When I inspected the car it had the original Bridgestone Potenza tyres fitted front and back. 

When I returned to pick up the car a few days later the dealer had replaced the front tyres with a cheap set of Chinese made tyres that have many bad reviews on line. When I questioned the dealer they claimed that they were trying to look after me and that they replace any tyres that don’t have a minimum of 3mm of tread left.

My question is are they able to do this ? I would have happily paid the difference between the cheap set and a good quality brand if they had of mentioned it to me. It’s soured my experience with this dealer or is this common place these days?