The FCC only has the power to assess fines, not collect on them. Plus any fines they do collect go straight into the general US Treasury (not into the FCC's budget) so there's no fiscal incentive for the FCC to assess, or not assess, fines.
Similarly, the FCC cannot enforce a shutdown, they can only collect information. The US Marshals have to enforce an actual arrest or confiscation of equipment. As you might imagine, the Marshals are mighty busy ever since 9/11, so it's harder for the FCC to get them to go make an arrest or seizure.
There's also an unspoken mandate that the FCC is
not a proactive agency. They are primarily a
reactive agency...meaning they have to receive a legitimate complaint before they'll act. Legitimate complaints are those from stations directly interfered with by the pirate (as defined by contour overlap) and from listeners living within the interference area. Everyone else is not exactly ignored, but much less weight is given to their complaints. In addition, all complaints must be made via snail mail as described here:
http://www.fcc.gov/eb/broadcast/interference.html
I'm not saying that to dissuade anyone from making a complaint. By all means, please write to the FCC...even if you're not "legitimate" as described above. You never know if your complaint will be the one that tips the scales into action...I've seen it before. Plus if dozens or hundreds of people complain, even if many of them are not "legitimate", the FCC will still take notice. Godcaster WSMA 90.5 was forced to install a main studio and actually staff it according to the rules after the Boston Phoenix helped champion a letter-writing campaign that generated over 200 complaints to the FCC about WSMA's request for a main studio waiver. Unfortunately it was a symbolic victory at best, but it still proves the point.
There is also the unfortunate reality that the FCC has to decide what issues are a priority for them every year...and pirate broadcasters consistently rank low on the priority "list". This "list" is determined largely by what Congress is breathing down the FCC's neck about. Also by whatever political leanings are coming out of the White House on the five head Commissioners. Usually it's more about wireless communications (cellphone/wifi) because there's a lot more money at stake, and therefore a lot more lobbying of Congress about it. When it comes to AM & FM, usually Congress & the WH are a lot more "concerned" (one could call it "pandering") about obscenity and indecency.