The law and source of the law with regard to besulos, virgins, (that they need not cover their hair):

It is explicitly stated in Tur and Shulchan Aruch, Orach Chayim chapter 75. See also commentaries on Shulchan Aruch, Even HaEzer 21:2.

We may say that the reason for the difference is: The verse "He shall uncover the hair of the woman" (see Kesuvos 72a) is stated with regard to a married woman. Most importantly, it is similar to that which is stated in the introduction to the Rambam's commentary on Mishnayos, where he states that matters that have been accepted from generation to generation going back to the times of Moshe Rabbeinu Alav HaSholom, cannot have divergent opinions.

With regard to the above, we see that this has been the practice of besulos from generation to generation. And according to R. Yishmael (Sifrei, Bamidbar 5:18) there is an allusion in Torah to the fact that besulos go with their hair uncovered.

We may say that the inner reason according to Nigleh is: Covering the hair is a punishment for the sin of the Eitz HaDa'as (Eruvin 100b), for she caused her husband to succumb. Thus it does not apply to a besulah.

Excerpt from Igros Kodesh, Vol. XI, p. 200