This relates to the frequent call by Plain English advocates: 'Avoid the passive'. Language Log has pilloried this before, and rightly so. Not every use of the passive in English is overuse, wimpy, wishy-washy or indirect. But what about the situation in legal English?
In Evidence class, the professor recently pointed out that in summation to the jury, "The evidence has not been disputed" is fine, but "The defendant did not dispute the evidence" is prohibited by the rules, because the defendant's right to refuse to testify cannot be questioned.
This use of the passive has more to do with tactics than style. I presume it means you could say 'The defendant's lawyers did not dispute the evidence', but that would sound a bit negative, as if one were to say 'Maybe they should have done - maybe he had the wrong lawyers'.
The second point Zwicky makes is a plain English point:
Meanwhile, in Legal Writing classes, the professors insist on Avoid Passive for briefs and memos. Here I see the long influential arm of Bryan Garner, whose The Winning Brief: 100 Tips for Persuasive Briefing in Trial and Appellate Courts (2nd ed. 2004) and earlier books on legal writing come down hard on the passive voice (and restrictive relative which and sentence-initial linking however, among other things).
Whatever one thinks of Garner's opinion on the English language, there's more to lawyers and the passive than this.
One piece of advice for lawyers drafting contracts is: It's better to write The landlord is to paint the exterior once every six years than The exterior is to be painted once every six years. Well-drafted contracts make it clear which party is to do what. Thus, for example, in The Elements of Legal Style, Garner writes that X shall give notice is better than Notice shall be given.
I'm not sure if the guys at Language Log Plaza have considered this real-world aspect. But maybe I'm over-sensitive because German contracts so very often use the passive and impersonal. What is the translator to do? I do not usually translate passives into actives in examples like these. But in other cases, I have to make a sentence active or personal for language reasons, and I have to be certain I use the right subject.