It is complicated to answer why a law is what it is. Judge Frank Esterbrook writes (in the forward to Reading Law by Scalia and Garner):
Every legislator has an intent, which usually cannot be discovered, since most say nothing before voting on most bills; and the legislature is a collective body that does not have a mind; it "intends" only that the text be adopted, and statutory texts usually are compromises that match no one's first preference. If some legislators say one thing and others something else, if some interest groups favor one outcome and others something different; how does the interpreter choose which path to follow?
I will provide the historical background leading to the current fair use statute and case law, but take from it what you will as to why it is what it is.
Fair use is a statutory defence provided by 17 USC 107. Its application is clearly demonstrated in Campbell v. Acuff-Rose Music, 510 U.S. 569 (1994).
Fair use was an "exclusively judge-made doctrine until the passage of the 1976 Copyright Act". (Campbell)
Courts had been finding exceptions for "fair abridgements" and other precursors to fair use as far back as under the Statute of Anne of 1710. (Campbell, citing William Patry's "The Fair Use Privilege in Copyright Law".)
This doctrine worked its way into US case law in the nineteenth century.
In Folsom v. Marsh, 9 F. Cas. 342 (No. 4,901) (CCD Mass. 1841)1, Justice Story distilled the essence of law and methodology from the earlier cases: "look to the nature and objects of the selections made, the quantity and value of the materials used, and the degree in which the use may prejudice the sale, or diminish the profits, or supersede the objects, of the original work." (Campbell)
There must be real, substantial condensation of the
materials, and intellectual labor and judgment bestowed
thereon; and not merely the facile use of the scissors; or extracts of the essential parts, constituting the chief value of the original work.
This early incorporation of fair use focused on whether something new was being created, or whether the "chief value" of the original work was being taken. This primary focus on transformativeness has stuck with with fair use doctrine until today. Campbell said (internal citations removed):
The central purpose of this investigation is to see, in Justice Story's words, whether the new work merely supersedes the objects of the original creation ("supplanting" the original), or instead adds something new, with a further purpose or different character, altering the first with new expression, meaning, or message; it asks, in other words, whether and to what extent the new work is "transformative". Although such transformative use is not absolutely necessary for a finding of fair use, the goal of copyright, to promote science and the arts, is generally furthered by the creation of transformative works. Such works thus lie at the heart of the fair use doctrine's guarantee of breathing space within the confines of copyright, and the more transformative the new work, the less will be the significance of other factors, like commercialism, that may weigh against a finding of fair use.
Campbell also includes the following statement of rationale for why Congress included section 107:
Congress meant § 107 "to restate the present judicial doctrine of fair use, not to change, narrow, or enlarge it in any way" and intended that courts continue the common-law tradition of fair use adjudication. H. R. Rep. No. 94-1476, p. 66 (1976) (hereinafter House Report); S. Rep. No. 94-473, p. 62 (1975) (hereinafter Senate Report).
There is no support for your separation of "parody, comment, and criticism " from "teaching, research, and news reporting", or that there are differing amounts of commercial use allowed for these types of work. The statute lists together "criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" as examples purposes. The statute requires that "whether such use is of a commercial nature or is for nonprofit educational purposes" be considered in all fair use cases.
There is also very little if any First Amendment rationale expressed for the fair use doctrine. The intersection of the First Amendment and copyright law is more clearly found in the idea/expression dichotomy.
1. Folsom v. Marsh full opinion text