It seems like legal texts should be extremely unambiguous, because they are produced by institutions who are presumably aware of the problem of linguistic ambiguity, and otherwise how are legal documents supposed to do their job?

That assumes that the linguistic ambiguity is bad for enough stakeholders.

A lobbyist who writes an amendment for a law doesn't always want that the politicians who vote on the amendment understand what it does.

An ambiguous version might also be a compromise that allows both sides of a conflict to avoid losing.