I'll preface this by saying this is all my own opinion:
FCC enforcement is to a large extent complaint driven. These complaints can come from other broadcasters (often the cause of busts of FM pirates), government agencies, and individuals. If you have not already done so, go visit
http://transition.fcc.gov/eb/ and take a look at the Field Issued Citations, NALs and NOVs section
http://transition.fcc.gov/eb/FieldNotices/ and read some of the entries. You will see that they typically begin with statements such as "The Seattle District Office received information that an unlicensed broadcast radio station on 98.5 MHz was allegedly operating..." and "This office received a complaint from a licensed telecommunications provider that it was receiving interference in the 450-470 MHz band from a radio transmitting device"
I find it difficult to believe that the FCC spends significant resources monitoring the HF spectrum looking for unlicensed broadcasts or transmissions otherwise in violation of FCC regulations, and taking enforcement action. I'd even go so far as to say that the FCC seems to tolerate a certain level of improper/unlicensed operation, so long as it does not cause significant harmful interference to licensed users, or get out of hand. Spending a few minutes on 14313 kHz will quickly confirm this.
As Andrew pointed out, things were very different in the 70s and 80s. The FCC had a large enforcement budget, and made use of it. Pirate radio was quite different back then. Not to get into the "back when I was a kid, I had to walk 10 miles to school, uphill both ways" mode, but pirate activity was much lower back then. You could go weeks without hearing a station. It was far too risky to run any sort of a regular schedule, or transmit for hours at a time.
Also as Andrew said, it is expensive for the FCC to send personel to make the bust. But roughly DFing a pirate is trivial with their equipment. Now, I'm going to speculate here, but it is entirely possible that the FCC maintains a database of known pirate operation locations. They could even cross reference this with ham license information, or even addresses of known pirate enthusiasts (following the likely theory that operators are probably active listeners as well), and come up with probable hunches as to who operates the various stations. Then, should enforcement actions be required, they're ready to act.
Based on what I've read and observed, if I had to make a guess, I would say that WEAK was busted due to interfering with utility (possibility military) comms.
I cannot say the same for The Crystal Ship bust. Certainly not with the same certainty. There's way too many coincidences and circumstantial evidence with that bust. There's a lot of utility and .mil operations on 43 meters. Poet could have run afoul of one of them. He was also operating near a MARS net. It could have just been his bad luck to have interfered with the wrong transmission(s). Here's the opening of Poet's NOUO letter: "The Detroit Office received information that an unlicensed broadcast radio station on 6815 kHz was allegedly operating in Lansing, Michigan." The FCC doesn't disclose who provided that information. But there's something interesting about the wording. Take it literally. The sentence says that the FCC received information that an unlicensed broadcast radio station on 6815 kHz was allegedly operating in
Lansing, Michigan. How would a utility or MARS station know that The Crystal Ship was located in Lansing? You can understand how a listener would know where an FM pirate was located, as they typically only serve one city. But a shortwave pirate? Maybe it is just how the FCC words their notice. Or maybe not.