I saw it happen lately a few times that players:
1. Build at teammates lines to steal creeps within their maze
2. Build at a teammates line to steal creeps right at their spawn (some towers are easily capable of it, like the 100g of the tank builder)
3. Steal leaks that would teleport to another players line by building in front of the teleport point.
I suggest
Do not build at another players area without his agreement to do so.
Some players actually help each other and share auras and all so it should be allowed.
WMO isnt supposed to be played with stealing kills, the new versions even were updated to reduce kill stealing due to high ranged towers.