Should I reframe and post this question on Law SE instead of ELU?
Should? Sounds like it could be on-topic, but I don't know any way to determine how valuable a question it is until it's actually posted for voting.
Amendment: I'd recommend reframing and posting to Law.SE something along the lines of, "Do italics have special meaning in legal documents?" or, "What style guides or rules, if any, apply to legal filings in U.S. venues?" The answer on ELU seems to have addressed the particular style guide you found. If there is an answer to the Why? question I imagine it's more likely to be known by legal experts, hence more on-topic here.