I've never had them build inside my ZOC, but I don't Non-aggression people too often either, so it may be something with that specific pact. Still, they will try to get that shard (by building outposts right outside your ZOC) because they think it as unclaimed. No worries though, once your city grows, the city's ZOC will overpower their outpost ZOC, and you'll have it under your control anyways. Basically, they are just borrowing that earth shard (and building it for you) until that happens. If you really want it fast, you should build your own outpost and reclaim it (you can build outposts nearby to push their ZOC until the shard becomes yours again). You can actually do that with all of their outposts BTW... it's cheap, but very effective.
