Chroma wrote:
Basically, it's fine to agree to that in friendly play, but in "strict" tournament play, each army, and all its constituent parts, are considered its own entity with no sharing to the opponent.
This. The army lists are written as the gates being part of the buying player's army. Even though every subsequent reference to the portals doesn't include the "in the army" qualifier, that isn't a good reason to claim that the context vanishes.
If you need a fluff reason, no craftworld knows all the webway. Even if they did, the webway has its own internal navigation issues. The craftworld could exactly where the "enemy" gates were and it might still be nigh impossible to get to them from the paths they know (at least in the time frame of the battle). If gate-hijacking was a real threat, the enemy would have guards in the webway and you'd just have an extension of the "on board" battle inside the webway.
