It is actually not that tough if you know what you are doing. A BTC address won't be enough to track down a user unless they fill in KYC or do something stupid. It all comes down to how smartly you utilize the tech behind BTC.
So you think there are a lot of people who are experts at not leaving digital footprints out there? If this were the case, the administrator would not have made the error that exposed his public IP when using TOR.
Why is this so surprising? BTC has been steadily rising in value and gaining popularity over time which is why more and more users around the world are embracing it. Also, I am happy that the FEDS actually tracked these pedophiles which is why I applaud them in this case.
BTC may be popular, but if you are doing or are going to do illegal stuff, does it make sense to leave any traces?