The use of the term "seize" implies that the Feds were able to get a hold of the coins without the owners consent. However, while no exact details have been released, the prevailing thought is that it was done through a sting operation where the individual willingly sent the bitcoins to the Feds and they are now keeping them, thus the "seizure."
Funny, but law enforcement uses the same terminology when they bust people using traditional cash in sting operations. Money freely transferred is still reported a seized because that indicates there is no intention to return it to anyone connected with the bust.
Do you have evidence the government wasn't able to go into his wallet and take Bitcoins without his consent? To the best of my knowledge neither the feds nor the suspect have provided details of how the Bitcoins were seized. It could have been simply keeping the Bitcoins used in an attempted illicit transaction (most likely), or the feds could have figured out a the password used by the suspect to open his wallet (least likely but still possible). Which ever the fact that the Bitcoins were seized has got to lower the confidence of those contemplating using Bitcoins for illicit transactions.