opponent intercepting throw-in pass

[mdray]

I know we've been round & round on this topic before but the sitch came up this weekend in a game. On a designated spot throw-in, the thrower passed the ball "diagonally" to his teammate inbounds. The defender reaches across the throw-in plane and intercepts the pass. Do you have a whistle there? (the defender did not reach across and touch the ball while it was in possession of the thrower and the defender did not reach across the plane until the ball had been released by the thrower).
I had no whistle on the play. But afterwards I was thinking that, if the offense had caught that throw-in pass on the OOB side of the plane, that's a throw-in violation. So if the offense isn't allowed to catch it there, why should the defense be allowed to? And isn't it only a T if the touching by the defense occurs while the ball is in the possesion of the thrower? I can't find a rule/caseplay to answer this.