Yea, it may be true for this one case that they are not looking for evidence so much as leads. The problem is this one case affects 100's or potentially thousands of others in the future, by setting legal precedent.

Based on news reports there are currently 100's of criminal cases and many prosecutors waiting to use the "FBiOS" once it is developed. If the DOJ's goal was strictly about terrorism there are alternative ways to solve this and Apple would likely be more accommodating, as it appears they have been in the past. But the DOJ is now looking to establish legal precedent within the courts so it can be used routinely and not just for terrorism cases.