The argument is simple:
1. Knowing the identity of a reporter is immaterial to the substance of the report. The reported comment, vote, thread, etc. is a violation or it is not. The status of that violation does not change if it was reported by Bob vs. Alice.
2. Knowing the identity of a report can only possibly introduce bias in how mods respond to reports. Mods should be objective.
3. There is no problem that requires knowing the identity of a reporter.