I see a lot about why people aren't allowed to practice law on behalf of another person, but I do not see any positive justification for allowing a pro se litigant to practice law on behalf of himself.
Is there a court case that decided pro se's are allowed to practice law on behalf of themselves? Because I would like to see the exact reasoning. I wonder if it is constitutional or based in common law or what exactly.
A summary is here: see for example Section 35 of the Judiciary Act of 1789: 'in all the courts of the United States, the parties may plead and manage their own causes personally or by the assistance of counsel'. In the same vein, Adams v. United States ex rel. McCann, 317 U. S. 269 held that "an accused, in the exercise of a free and intelligent choice and with the considered approval of the court, may waive trial by jury, and so, likewise, may waive his constitutional right to the assistance of counsel". In Faretta v. California, 422 U.S. 806 it is held that this extends (via the 14th) to state courts.
As to the expression "practice law", maybe the meaning of this expression is taken to be self-evident, though clearly it is not. Statutes that prohibit that act for non-lawyers generally specify things such as "for another person" or "holds himself out as entitled to practice law".
Your question is a little bit backwards. It's not that there is a ruling to grant people a "right" to "practice law" on their own behalf; rather, there is an absence of a ruling denying them the right to speak for themselves in court. Courts and similar legal hearings existed before there was a legal profession; in the beginning, one pleaded one's own case because that was the only way. As laws became more complex, specialists arose who by virtue of their expertise acquired the right to act on behalf of a party to an action, that is to say, to act as that person's attorney at law.
While some jurisdictions may have decided to remove litigants' ability to have access to the court without the aid of a specialist (that is, of a qualified lawyer), the United States has not chosen to do so.
I will echo the other answer in saying that the "practice" of a profession generally means the provision of services to others for some consideration. Representing oneself in court is no more the practice of law than is bandaging one's own wounds the practice of medicine.
I should add that there are some exceptions where a specific party is not allowed to represent himself in U.S. courts. The exceptions that come to my mind are where (1) the party is found to be legally insane and consequently unable to intelligently conduct his case, (2) the party's gross misconduct repeatedly disrupts judicial proceedings, and (3) a court rules that the party is a vexatious litigant (based on his or her pattern of frivolous filings).
