I assume they had some contract to spell out the legal level of access that Huawei had... the level of technical access would presumably have been exactly the same had they outsourced the maintenance to a western company, the legal protections would presumably just have more weight in that situation.
The worrying part is that the owner of the network apparently lacked audit logs of any such technical level access. That's still just as much of a problem with insider threats if the company manages the network themselves?