I think everybody (well, almost) is more likely to view a person they dislike as having NPD. It's a lot easier to write off somebody's behavior to narcissistic selfishness than to try to understand its real motivations. A lot easier to stick them with an unflattering psych label. A lot easier to decide they're crazy than to see if they're justified.
Also, the average person's mental definition of narcissism is likely far less strict than the DSM-IV, so a higher percentage of the population would fit it.