Most dictionaries say the word "guideline" is USUALLY used in the plural.
Q1: Why is that? Because a document called guidelines contains line"s" (sentence"s", not only a line, a sentence)?
Q2: It's not incorrect to say "guideline", without "s", is it? I find a lot of "Guideline for something" (without "s") written by native speakers of English on the Internet.
Example 1: Practice Guideline for the Treament of Patients with HIV/Aids (by American Psychiatric Association)
Example 2: Guideline for Submitting Documentation for the Stability of Human Drugs and Biologics (by Center for Drugs and Biologics, Food and Drug Administration)
Or, is it a kind of mistake native speakers make if they are not very conscious of the language?
It seems that an official document detailing the guidelines for something can be called a 'guideline'. It must be taken as a whole. If bits are left out, it still contains guidelines, but it's not the official Guideline.