Here's the EPA's text (off their website, pulled 5/6/2003) regarding repair of leaks and topping off of leaking systems:

What are the leak repair requirements?
EPA regulations do not dictate any particular service, as long as a technician is certified to work with refrigerant and any recycling equipment he or she uses meets EPA standards. EPA does not require that leak repair be performed before refrigerant is charged into a vehicle, although certain states and localities may require such repair. In addition, EPA does not require that the refrigerant be evacuated and cleaned prior to recharging the system with refrigerant. In other words, EPA does not require evacuation and recharge, and does permit top-off with the same refrigerant, in motor vehicle air-conditioners. If you are unsure about any EPA regulations governing auto air-conditioning, call the Hotline number listed above.

Obviously, the EPA does not require leaking systems be reclaimed, essentially stealing refrigerant from the consumer. I don't know why any technician would say anything different.

- JimY