It's fun the first few times. Maybe. But after the twelfth or thirteenth yahoo waltzes into your realm, makes himself a pain in the ass, then runs off to buy your ally's region just to "start some fun", you just boot the jerk to the curb, and move on. Alternately, the guy joins, says nothing, but immediately heads off and buys the region, then sends some annoying message about corruption, yada, yada...
The problem with these features is that they get used *way* too much, especially by some families who make it a habit. It's that one-click action that you can do without any interaction with other players, but you know will cause lots of problems. They just get used too much. Everyone wants to be that unique, special snowflake whose case is treated differently, and whose tired lies everyone plays along with.
Except you're not. You're just not that special snowflake. It's the same tired crap we've seen time and time again. Probably dozens of times. And quite a few people are just tired of playing along with it, especially when you know that the guy did it just to troll your realms.
So, if you want people to play along, then stop doing it every other friggin' week. Make it a *really* rare event. That way people can look at it as a "Holy !@#$! Look what happened!" and not "omfg... again?!"