Why does the Mishnah (65a) refer to a person who asks a Ba'al Ov to elicit information from the dead as a "*Nish'al* b'Ov" (literally "one who *is asked*..."). The verse calls such a person a "Sho'el Ov" (Devarim 18:11), one who *asks*!
There are two verses which prohibit turning to an Ov for information. One is the verse that you mention, which refers to "Sho'el" Ov (Devarim 18:11). The other is in Vayikra 19:31, which says "Al Tifnu El ha'Ovos" -- "Do not turn to the Ovos." According to Rashi on the Mishnah here, the verse in Devarim refers to the person who conjures up the dead with the bone, while the verse in Vayikra prohibits a person from asking the conjurer to elicit information. According to Rashi, it is obvious why the Mishnah uses the word "Nish'al" -- it is in order to differentiate between the conjurer himself (to whom the Torah refers as Sho'el b'Ov) and the person who requests the conjurer's help, to whom the Mishnah refers as Nish'al.
TOSFOS (DH v'ha'Nish'al), however, argues with Rashi on this point and asserts that the verse in Devarim, which refers to Sho'el Ov, is discussing the person who asks for the conjurer's help, which is the same person the Mishnah refers to as "Nish'al." According to Tosfos, your question remains.
Perhaps Tosfos will explain that the Mishnah uses the more passive term, "Nish'al," in order to emphasize that the person who asks for the conjurer's help does not transgress the prohibition of "Al Tifnu El ha'Ovos" (which would put him into the category of Sekilah; see Tosfos). That is why the Mishnah changes the term from "Sho'el" to "Nish'al."