it's pretty low because they need to have probable cause in order to charge you with anything. they also need to have more than just an email address to track you down (legal name, phone number, address). the less of that they have, the more barriers they must deal with in order to chase you down.
in response to the scenario marikod posted about offering you a deal to testify against her, it would actually have to go like this:
1. they would have to identify you correctly, find information either in an email or through information she has provided that gives them probable cause to arrest you (meaning any indication that you committed a crime).
2. they would have to find you, arrest you, and charge you with a crime.
3. you would then have the choice of pleading guilty immediately, taking it to trial, or negotiating with the prosecutor so the court can avoid trial. in order for them to win in a criminal trial, the jury (or judge, if a bench trial) must agree that the prosecution has proof beyond a reasonable doubt that you committed the crime. often, cops will arrest someone and the state or city will prosecute without having any real evidence, which is why so many people take plea deals.
often, a lot of "negotiating" happens in the cop station, with no lawyers or prosecutors present. you need to know that the only one who can offer a plea is a prosecutor (NOT a cop, they have no legal power once they've arrested you) and the only one who can approve it is the judge.
the cops rely on people's lack of knowledge of the legal system to coerce them into giving information that will allow them to gain "probable cause" to arrest and charge others, and to coerce them into testifying against others. there are lots of ways to fight a charge other than testifying against someone. no matter what a cop tells you, they CANNOT guarantee you will get off or walk away scot-free just for giving information. cops are given a legal right to lie. also, if cops really had enough information on you to successfully charge you, they wouldn't even bother talking to you.
if providers and clients have any respect for each other and this profession at all, they would make sure they understand their civil liberties, the legal system, and the actual laws in their state and city. and they would NOT snitch on each other!
often, people feel like they cannot handle the time, money, effort, stress, bad publicity it takes to go through a legal process that involves lawyers, negotiation, filing motions, and is longer than a few weeks. pleading is okay to do when it only affects yourself, but pleading in conjunction with a deal to snitch is unexcusable.
http://www.aclu.org/police/gen/14528res20040730.html
http://www.midnightspecial.net
providers, check out your local Sex Worker's Outreach Project, they often have a legal number and resources. they are located in most major cities (SF, NY, Chicago, etc.).