Why did tradehill decide not to cooperate with this blacklisting thing? Was it the moral hazard?
Because they shouldn't. Mt. Gox is wrong.
There is no way to know the party in possession of the coins is the thief.
Thief steals coins.
Thief says I will give you 100K BTC for an equivalent amount of gold.
Thief has already profited, you have given up gold and are holding coins. If those coins are worthless (or worth less in value) because of some merchants belief they have a God given right to be the Bitcoin Police then it undermines the entire system.
TL/DR version.
A bitcoin is only worth a bitcoin by consensus. People deciding some bitcoins are worth more than other Bitcoins is not only idiotic it undermines the entire system.