Cynical scenario: the trusted-source catspaws have been reliable sources in the past, and will seemingly continue to be reliable sources in the future. This is their “price” for giving up reliable information for free—the ability to be used as a mouthpiece by their organization. Bloomberg accepts this deal, because they’re so useful otherwise.
Noble scenario: the sources didn’t want to transmit this information any more than Bloomberg wanted to receive it, but they were forced to (by, perhaps, a National Security Letter; or just generic “we’ll hurt your family” coercion); Bloomberg knows this, and doesn’t want to hurt the sources either, when it isn’t their fault (and either don’t know who coerced them; or do know, but know that it’s an actor powerful enough that they know that reporting the source’s coercion would just mean the source’s untimely death. It’s not like a newspaper is a government that can offer witness protection.)
The flip side is that on a story this big, they shouldn't publish if they won't be able to burn their sources if it turns out they're being played. If a source is too valuable to challenge, they're not worth enough to publish.
That requires having a degree of certainty supporting any attempt to name names. If you're not certain they lied to you, then your new story is potentially libel. For example, let's say they were given information they were certain was correct, and they handed it to a journalist in confidence. They're a trusted source. You both got duped. What's the value in burning them publicly, especially if you're going to do it by calling them a liar and exposing yourself to risk?
There are cases where the press really needs to take people to task but the amount of secrecy and ambiguity in this story makes it seem like a case where it can't (or shouldn't) be done.
If you lie to a newspaper to get fake news published, you no longer have any right to remain off the record.
That's what a newspaper with integrity would do.