Mueller's report was not for the public, it was for the AG. Barr is the AG. It was his report.
It was in the public interest to know what the report contained, so he summarized the findings, then made extra effort to release the report, with less than 1% redacted, after concerns about context were raised.
It should also be noted that it is NEVER the prosecutor's job to exonerate. Mueller's text regarding that point was entirely political and anomalous from professional behavior.