I don't think the purpose is to stop landtrading, so much as it is to make it less powerful.

Also, to the earlier questions on ghost acres % for dict: Dict's get a 33% bonus to the 43% ghost acres that other govs get. 0.43*1.3 = 0.5719, so Dict's get 57.19% ghost acres, compared to other governments 43%.

Easy way to calc units to be sent to gain max ghost acres:

0.068*(0.5719 for Dict, 0.43 for others)*(1 for SS, 1.5 for PS)*(1.2 for Tyr, 1 for others)*MStrat%*DefendersLand*YourLand/20

In practice, the amount of units you need to send will be slightly lower than this due to the defender having CS and possibly unbuilt acres, and due to NW not being matched to exactly 95%, but it's a quick calc to be sure that you are sending enough units to guarantee full ghost acres.