I "think" that any operation that takes monies for connecting up to a "sealed" fridge system, should be, nowadays, required by law to evaluate the sealed system for leaks, repair, pressure test, vac out and reload with the predetermined design weight of liquid. It gets worse, for systems carrying more than maybe 10Kg (guess), the load has to be removed weighed and reloaded once every 12 months.
Having said all that, the "Mickey Mouse" regulations covering the refrigerant in car systems has been written so that it is "your" fault when the charge is lost - so no one is held accountable for unknown losses. That means that the licensed handlers of this liquid can claim that your system was empty when you submitted the car for repair - because the law can not touch you - so the handlers can cover up nicely any losses - that is why the likes of KwikFit can get off with just "topping up" without anyone being able to prove that it happened. Proper A/C places will have a different way of working and will normally just remove the charge completely. I think that the drier/receiver only actually needs replaced if the system has be open to atmosphere for some time - like a broken metal pipe etc. I still think that its a good idea to replace it after maybe 4 years - but that adds a bit of money to car servicing - so its avoided, I'm shocked to hear that Seat claim that not changing it every year would invalidate the warranty on the system - that means that it and other jobs like timing belts should be mandated in the service book.