The use of the term "white trash" is racist.  To figure out why, consider the lack of the term "black trash".

(Hint:  Try organizing the following terms hierarchically*:  white trash, white person, black person.)

*˙ʇs!ɔɐɹ ɐ әq ʎɐɯ noʎ 'uɐɔ noʎ ɟ!