If they know people wan't prostitutes they will set up fake prostitution stings , if they know people want drugs they will have undercovers selling drugs, if they know people want to launder their illegally gained funds they will set up sting ops that would attract those type of people.
I don't think this is an attack on bitcoin so much, just another example of police going undercover and offering illegal things/services and then see who bites.
What "illegal things" were offered by the undercover lawn enforcement orificer to the bitcoin seller? It was just an exchange of bitcoin for dollars. It's clearly just another case of flat out persecution. The only criminals here are the costumed thugs who kidnapped these men at gunpoint and locked them in cages for absolutely no justifiable reason. Those men harmed no one. There is no victim here. There is no loss and no injury and no fraud. Therefore, there is no crime.
You missed a key part of the story where the undercover agent told the bitcoin seller that he was going to use the bitcoins to purchase stolen credit card information. Thats the key part in this case, and probably the only reason an arrest was/could be made. It's one thing to exchange bitcoins for cash, it's another thing to do it when the buyer specifically tells you he intends to use the bitcoins for illegal reasons and you STILL go through with the trade.
"According to court documents, the agent told Michelhack that he wanted to use the Bitcoins to purchase stolen credit cards online. "
http://krebsonsecurity.com/2014/02/florida-targets-high-dollar-bitcoin-exchangers/
Like I said earlier, it's a legal form of entrapment and it's something law enforcements do for all sorts of illegal things. The issue here isn't so much bitcoin , but selling bitcoin to someone who told you they would be used for a crime.