Because lie detection is an inexact field searching for any data point that might be helpful.  I do not say this casually, but because 1) using the P300 as a measure of lie detection is not supported by replicated research and 2) the P300 is not fully understood - although there is a great deal of excellent research in using this data point.  Therefore the use of the P300 in lie detection cannot be done honestly by anyone with an understanding of this event-related potential.  The best reason I can think of for using the P300 for lie detection is that those in the field of lie detection are satisfied to apply measures they do not fully understand.

 The use of ERP and the P300 Is yet another area of study more accurately addressed as the instrumental assessment of credibility.  Use of the term "Lie Detection" is factually a misnomer in that the person being assessed doesn't have to reply verbally, which begs the question if a person isn't required to speak then how do they lie?

Measuring the P300 or other physiology using a polygraph instrument is a structured interaction conducted to see if the person being subjected to assessment has resident, and is withholding, a significant autobiographical/episodic memory related to the issue being addressed.  There are very few scientists currently interested in this area of study, perhaps due to the apparent belief that everyone is an expert regarding

There are very few scientists currently interested in this area of study, perhaps due to the apparent belief (see the above flippant response) that everyone is an expert regarding lying because we all do it. However, serious study (i.e., using the scientific method) can be traced to a study conducted in 1969 (Bersh, P. (1969). A validation study of polygraph examiner judgments. Journal of Applied Psychology, 53, 399-403.) with a great deal published since then. If you would like more information on anything in this area please ask, if I do not know the answer, I know someone who does.

If not interested perhaps Lynn would be kind enough to point out an "EXACT" field of research that would be of interest. 

to make up your own opinion:

Allen, J. J., Iacono, W. G., & Danielson, K. D. (1992). The identification of concealed memories using the event-related potential and implicit behavioral measures – a methodology for prediction in the face of individual-differences. Psychophysiology, 29(5), 504–522.

Farwell, L. A., & Donchin, E. (1991). The truth will out – interrogative polygraphy (lie detection) with event-related brain potentials. Psychophysiology, 28(5), 531–547

Meegan, D. V. (2008). Neuroimaging techniques for memory detection: Scientific, ethical, and legal issues. The American Journal of Bioethics, 8(1), 9–20

Meijer, E., Ben-Shakhar, G., Verschuere, B., & Donchin, E. (2012). A comment on Farwell (2012): Brain fingerprinting: A comprehensive tutorial review of detection of concealed information with event-related brain potentials. Cognitive Neurodynamics, 7, 155-158.

Article A comment on Farwell (2012): Brain fingerprinting: a compreh...