Depending on details, it may not be an unreasonable request. The question is more one of trust: do we trust the FBI that it's a reasonable request?
This is why all the stuff like the activities the Snowden leaks demonstrated or Trump's idiotic harassment of the press through the DOJ are so harmful far beyond the direct harm they did: they justifiable and seriously erode trust, and then there is a serious case like this and "trust us" no longer carries any value. A sad state of affairs where everyone loses.
No. Under no circumstances is it acceptable for governments to ID readers of a news article. Such an act is a direct attack on the freedom of speech, freedom of thought, and freedom of the press. IMO it very clearly crosses a line on what law enforcement can and cannot do in a democratic society.
Note that law enforcement has (over)broad powers for investigation at their disposal already, and they were able to find the suspect "through other means." There was no justification whatsoever for the subpoena.