Because using Tor or Freenet is not a crime. There's a chance you could be investigated - we've definitely done that for things that weren't criminal, and I wish we wouldn't. Just for using a legitimate anonymizing service, though, the chances of prosecution are remote. What is the prosecutor going to say that a non-asleep defense attorney can't tear apart? They'd have to charge you with something specific - and for that, they'd have no evidence (the only sector of US law enforcement that tolerates locking people up on no charge and throwing away the key is immigration - and again, I seriously wish we wouldn't do that).
The worst it could do is attract law enforcement attention that could either lead them to something you've been doing - and there are enough freenet users that they can't afford to investigate every random user.
Because using Tor or Freenet is not a crime
Yet. But new laws and treaties such as SOPA and ACTA are conceived every day...
They'd have to charge you with something specific - and for that, they'd have no evidence...
We're almost at the point now where you can be indefinitely detained on mere suspicion of being a terrorist. And if you happen to have a suspicious-sounding last name, and you happen to be using suspicious services such as Tor to communicate with your relatives in a suspicious country such as Syria -- then you're acting very suspiciously ...
Referring, of course, to the proposed U.S. legislation which could cause severe damage to the Internet—at least, that's what a lot of people are saying. See, e.g., this Open Letter From Internet Engineers to the U.S. Congress (the first signatory listed is Vint Cerf). On Wikipedia, people including Jimbo Wales are discussing strategies as extreme as blanking the entire site (except for an explanatory message) to get people's attention, and thereby perhaps incite them to action, such as calling their Congressional representative.
I just happened to find out about all this a few hours ago, being someone who tries to avoid distractions like most kinds of news, so possibly others here with similar habits will appreciate having it called to ther attention. Or possibly they won't. But to those of you who possess relevant kinds of expertise:
(I think this subject can be discussed without political advocacy, in which I am mostly not at all interested anyway. It just looks like a practical problem to me.)
Edited to Add: I forgot to include a fourth bullet point:
It seems to have been assumed by many commenters, nevertheless.