They always wipe the memory of everybody who is not a member of MIB. James had seen an alien, had seen alien weapons, was shown a location of a store operated by an alien who was selling illegal alien weapons, etc. Even though James was being brought in for testing, he was not yet a member of MIB who could be trusted with this information.
Wiping his memory also leveled the playing field with the other people being tested and insured the validity of the test results. For example, the test where they go into a shooting range which was purely a test of their ability to ignore appearance when assessing danger.
Finally, wiping James's memory allowed for the rest of the scenes where the truth is revealed in the station.