The only organization with internationally-recognized authority to name asteroids is the Committee on Small Bodies Nomenclature (CSBN) of the International Astronomical Union (IAU). We Spacewatchers, as members of the IAU, subscribe to and recognize that authority. In the case of asteroids with orbits good enough to receive permanent numbers from the IAU's Minor Planet Center (MPC), names may be suggested by the discoverers. Suggested names are reviewed by the CSBN and the approved names are announced periodically by the IAU. A description of how objects are named can be found in:
Public Naming of Planets and Planetary Satellites (PDF)
While it is true that Spacewatch DETECTS hundreds of asteroids per observing night, detections on single nights are inadequate to establish accurate orbital elements. That accomplishment requires many more observations spanning years. So only asteroids that are repeatedly re-observed (deliberately or serendipitously) finally receive permanent numbers. Amateur astronomers regularly observe asteroids, and their work is extremely valuable in increasing the accuracy with which asteroid orbits are known.
As of 2000 Sept. 12, Spacewatch had been credited with 231 asteroids that had advanced to the stage of receiving permanent numbers from the MPC. Spacewatch has suggested and received approval on the names of several asteroids. These names are usually selected from among our scientific colleagues, especially those who have worked in the discipline of asteroid science. Nevertheless, we, as discoverers, do not have direct naming rights, and must be careful not to PROMISE in advance that the IAU will name an asteroid after any specific person, because every nomination is voted on case-by-case by the members of the CSBN.
This constraint is not an explicit prohibition against naming asteroids after generous philanthropists, but before any observers start offering asteroid names for money, the search community should agree upon some ground rules so that different groups will not start competing with each other for the "price" of an asteroid name. That kind of auctioneering of the process of naming celestial objects might discourage the IAU from cooperating.
One could also get into complications like what type of asteroid is being "purchased". Asteroids have to have very well-determined orbits before the IAU will name them. It is a great deal more work to establish orbits of some classes of asteroids than others. How are we to take into account in the "price" the number of observations, the span of years needed to establish a numberable orbit, the observers other than the original discoverer who made vital recoveries of the object before its orbit was accurately known, and the computational creativity of the MPC in linking observations of the same object made many years apart?
Offering names that are NOT endorsed by the IAU would not be fair to the "customers". For background on this issue by analogy with star names, see
Buying Stars and Star Names
The implications of naming asteroids after contributors on the tax-deductible status of their contributions has not been addressed here, because we are not lawyers.
This is a complicated issue, and that is one reason Spacewatch has not been selling asteroid names.