Also, laymen are much more likely to simply cargo-cult a style without understanding the purpose or origin (e.g. if copy-and-edit is the root cause, you would s

Also, laymen are much more likely to simply cargo-cult a style without understanding the purpose or origin (e.g. if copy-and-edit is the root cause, you would still expect that people who are instructed to write laws from scratch would mimic that style. I am pretty confident that lawyers drafting contracts will maintain a style because of copying: if a wording has been 'proven' before then it's lower risk than trying to word it more clearly yourself). I don't think it makes for super solid evidence for the hypothesis (It doesn't contradict it, though).