From the MHRA Style Book, 4th Edition (London: Modern Humanities Research Association, 1991] (p. 30):

It is the responsibility of an author to obtain permission for the quotation of any copyright material if such permission is necessary. Normally it is unnecessary to seek permission for the quotation of brief passages in a scholarly work [...] In general, it may be said that the length of the quoted passage and the use to which it is put should be fair to the author and publisher of the work quoted in that nothing is done to diminish the value of their publication.

These are the guidelines for us researchers who are not at all legal experts and still need to use quotes and refer to other people's work in our own. I think the key phrase here is "the length of the quoted passage and the use to which it is put should be fair to the author and publisher". We are using the material here for learning purposes, and as far as I can see, people who use extraneous material on this site provide sources and references alongside. Both of these seem to me to be quite "fair" practices.