I sure hope you are not a lawyer for the defense of someone being stalked.
A bit more to the point: yes there are reasons to harass players or associations, but not rarely this is just out of spite, and/or because the reason is indeed as you described: "the other side should have invested more".
For some people, and indeed some associations (I have come to know them) that really is enough. Which not seldomly evolves into: "if we can outspend you, you are not investing enough"
Sorry but no, there is no reason to harass anyone for any reason.
If things go wrong with a player or an association, just ignore it or go somewhere else. No need to harass.
As for breaking majorities, it is indeed not against the rules of the game, but then again, if it is as revenge, it is pretty pathetic.
One party might feel harassed but there is NO RULES against owning a majority anywhere on the map provided you are connected to said facility per RN Rules. I have people that chase my majorities, and if you want to keep them, one of the best ways IS to invest heavily and make it not worthwhile, the behavior will eventually subside, you can also just move on to another area, but if they are determined then they will watch your mapping and follow (once they get bored they will move on, but how long will that take)? I really don't like people being able to see my map or my trains, but it's all part of "strategic playing" i guess.
For example, if a person is pulling for Titles, and they are in an assoc that is ranked higher, is it harassment or strategic to break the majorities of the goods they are trying to win the titles for by driving up the Wait times causing them to not get the titles and less pps awarded to them and their assoc??
Harassment in online games has more to do with someone is making negative posts and continues to do so repeatedly, then you can have a case for harassment.
Stalking is pretty hard to do, and would involve the Police not RN CS.
This is just how I view it, everyone else is completely free to have their own opinions.