Listening and watching is not a form of aggression. A state have right to listen and watch other state actions from their territory. If a foreign traffic is passing through your state territory, your state has rights to look into it.
On top of that, it is a tool, which can be used or abused. It can, for example, be used to avoid aggression. Just like talking to a high military leader of a hostile state can be used for 'good' or 'bad' purposes.
