The fact that someone renounces their citizenship is probably enough to deny them reentry.  in any case there is no obligation to give a non-citizen a reason.  Just like if I walked up to a another country's embassy and started demanding things.  They owe me nothing.

As for fears of terrorism, etc., threats don't have to be rational, just perceived.  many more people die on the highways than on planes.  When a plane goes down it is a huge issue but that many people die every weekend on the highways. 

if some government employee is faced with the decision of blocking entry to Ver or letting him in they are obviously going to choose blocking him for any one or more of the reasons I listed.  That government employee has nothing to gain by approving his entry and will face problems if something goes wrong.