I am shamelessly borrowing this from someone who posted it on another forum I frequent:

DEA used the identity of a woman put on probation to setup a fake Facebook profile with photos to try and ensnare her drug connections. As you can imagine, this raised a lot of eyebrows, including the woman who had no idea this profile was made. The Justice Department is defending the DEA Agent's right to engage in such tactics.

Washington Post's Radley Balko fleshes out the issue more in his column.

Consider what the federal government is arguing here. It’s arguing that if you’re arrested for a drug crime, including a crime unserious enough to merit a sentence of probation, the government retains the power to (a) steal your identity, (b) use that identity for drug policing, thus making your name and face known to potentially dangerous criminals, (c) interact with those criminals while posing as you, which could subject you to reprisals from those criminals, (d) expose photos of your family, including children, to those criminals, and (e) do all of this without your consent, and with no regard for your safety or public reputation.
Talk about a fucked up abuse of power. It's like if you put a wire on someone without their knowledge that makes it look like they were complicit in ratting out their fellows. But it's the Justice Department defending the agent that is the real issue.

So just a reminder kiddies. If you get sentenced to jail time, you better hope you dont know anyone the government is interested in tracking down, cause they might just pretend to be you while you are locked up in an effort to take down your known acquaintances.

But remember, it's only illegal when the other people do it.