How did he know? I mean, of course there are cases where your hand might be made public at the outset (Menagerie, Shanty Town, prior opponent's Cutpurse/Bureaucrat, etc. etc.) or what you are drawing might be public (Farming Village, Scrying Pool, etc. etc.). But I'm asking in this particular case, how did he know that the target was not in your hand at the start of the turn? Watching the backs of the cards to see which are the ones you have been holding versus have drawn?
Clearly someone missed the rule where Throne Room has to be the first action you play on your turn. It's in the official Dominion Tournament errata.
I mean, it's like you're saying that you can use Chapel to trash all your Copper. It's common sense that that card is for getting rid of curses. You have to understand, this game is unbalanced by design. That's why we have official Tournament Rules to make things fairer to everyone. Can you imagine if you could play Throne Room any time you wanted during your turn? I mean, that's just one step away from playing Throne Room on other people's turns. I'm going to Throne Room a Militia right after you play 4 Smithies and make you discard all those cards you just drew! Then what good would Smithy be? It's only good in the first place when you can chain them into each other, since if you don't use the Tournament rule about comboing cards of the same name, you'd need way too many Villages to play them, and there's only 2.5 or less Villages per person! (Depending on whether your division plays 4, 5, or 6 players.)
Plus, can you imagine how ridiculous it would be if someone could play a Throne Room... ON a Throne Room? You could play Militia FOUR TIMES.