At worst they are wrong and committing something close to libel.
At best they are doing something unnecessary (given they can just alert the mods) which generally leads to a lot of undignified and painful accusations and counter accusations.
Of course cheating should be taken very seriously. But those of us who are a little uneasy about a lack of evidence being posted in decided cases (Exy's in particular) would perhaps feel a little better if action was taken against those who seem to be fuelling a public witch hunt.