It depends. In some areas, local and state law says professional technicians must fix known leaks before they can legally recharge your A/C system. This is the case in Florida and Austin, Texas.
Federal law, however, does not yet prohibit the recharging of leaking A/C systems. But that may change.
The primary reason why they're so serious about fixing leaks is to stop the loss of R12 into the atmosphere. R12 is an ozone-depleting chemical that will no longer be produced after the end of 1995.
In the past, most people would simply have a few pounds of refrigerant added to their system if it was leaking -- which was usually enough to get them through the summer unless the leak was really bad. Then the following summer, they'd do the same thing again. Meanwhile, zillions of pounds of refrigerant continue to escape into the atmosphere and work their evil against the Earth's protective ozone layer high in the stratosphere.
By banning the sale of refrigerant to noncertified individuals, the Environmental Protection Agency eliminated easy access to refrigerant for most people. So now you have to take your vehicle to a professional for service. The professional technician is then supposed to identify any leaks and encourage you to have them fixed before he recharges your A/C system with more refrigerant. In practice, some individuals may tell you they can't (or won't) recharge your A/C system unless you have a leak fixed. They might be able to refuse you on moral grounds, but not legal ones unless you live in an area that has such a law on the books.
Since 1993, all service facilities that work on automotive A/C systems must have two things: a technician certified in refrigerant recovery and recycling procedures, and the proper equipment for doing so. The EPA wants professional technicians to do this because (1)it eliminates the unnecessary venting of refrigerant into the atmosphere (which was a common practice before all this came about), and (2) to extend the existing supplies of R12.
Approved recover and recycling equipment does a fine job of cleaning the refrigerant so it can be used over again. As long as the equipment is maintained and operated correctly, you should have no fear of recycled refrigerant. The Society of Automotive Engineers (SAE) has established minimum quality standards for purity and moisture content to assure everyone that recycled refrigerant is good refrigerant.