One topic in social psychology concerns behavior reactions when strongly held irrational expectations are disconfirmed; the beliefs may paradoxically strengthen. Remarkably, rules of grammar—such as our topic, the that – which rule—can be the subjects of these strong beliefs. Attorneys sometimes worry that violating one of these imaginary rules of grammar will prejudice judges against them, but believers—as we'll call those who contend the that – which rule governs grammar or usage—don't readily notice disconfirming evidence, and a believing judge won't notice yours. The proof is that when confronted with disconfirmation, believers express shock—not at their own credulity but at the enormity committed upon the English language.
Legal-writing authority Wayne Schiess, I was surprised to learn, strongly upholds the that – which rule, forbidding the use of which without a preceding comma to start a relative clause. (See http://tinyurl.com/yamenoc and http://tinyurl.com/ycz7hvk.) A reader brought Wayne's attention to the violation of this supposed grammar rule in Uniform Commercial Code section 2 – 714:
Where the buyer has accepted goods and given notification (subsection (3) of Section 2 – 607) he may recover as damages for any non-conformity of tender the loss resulting in the ordinary course of events from the seller's breach as determined in any manner which is reasonable.
Wayne's informant stared at UCC, § 2 – 714, most of the day, but he apparently never considered that his discovery controverted the supposed rule. Wayne agreed with his reader about the supposed which error; that should replace which, Wayne instructed, and with Wayne's help, I understand what drives this usage convention. When which appears without preceding comma, believers feel uncertain about the writer's intent, the uncertainty arising from English grammar's reliance on the comma to distinguish nonrestrictive clauses, hence on the omission of a comma to distinguish restrictive clauses. Since it's inherently harder to avoid omission errors than commission errors, the believer wants the added security afforded by confirming the comma's omission.
The rule provides the reader with greater confidence that the author intended a restrictive clause and didn't carelessly omit a comma, but the reader is unlikely to notice and question your meaning unless you err. Believers (probably a small minority of legal readers) derive a sense of security from authorial obedience to the that – which rule only when they are made insecure by deliberately applying the misguided rule.