Well let's suggest a scenario. Your faction claim a 10x10 (100 chunk) claim in the overworld and there is no sign of a base on top of the land. How do you know where to cannon in the 10x10? If you try to do this legitimately , you'll end up having to go through each (1x10) segment one by one at different y levels, which is ludicrous.
Or if there is a base, how do you know where to cannon? It's not unfair if you're raiding a big base with many separate watered boxes and you don't know what you're shooting at.
Furthermore don't impose rules you can't enforce. World download is a client side mod and can't be detected by the server at all. This reminds me of the rule not to ban evade on Discord. It's very easy to break the rule but impossible to 100% prove the rule was broken, since you can't get IPs or anything to identify users logging in from the same IP address on Discord. It's an unenforceable rule. You can look really suspicious but suspiciousness is not evidence.