Currently, the way that spells/abilities are set up fall into one of three categories: Self/Party Members/Anyone. Therefore, we can’t just make an adjustment to allow something to reach alliances only. If we were to do this, it would have to be done so that the spell applies to "Anyone."

In that case, we have to worry quite a bit about harassment and griefing that could result from that change. For example, when players have to be visible to interact with something (like a door), someone can cast invisible on them to grief them repeatedly. Secondly, players would be able to disrupt someone who is trying "pull with sound aggro" by casting sneak on them before hand. Furthermore, casting invisible on someone else and making them disappear without their consent is a major point of harassment.

These reasons are why we haven't moved to change the properties of Sneak/Invisible beyond their current party member limitation.