This is because you enabled scanning for PUPs and this is off by default (exception being a boot-time scan) as most users aren't aware what a PUP is or what to do when one is detected:
PUP = Potentially Unwanted Program - See
http://searchsecurity.techtarget.com/sDefinition/0,,sid14_gci1066761,00.html. Not included in this definition are tools which can be used for good or evil (as in this case), some have been legitimately installed for a specifically good purpose, but could have been unknowing installed for a malicious purpose.
So essentially it isn't a false positive when classified as a PUP as avast can't determine the intent, did the user install it, do they know what it is for and is it legit. If the users can answer yes for all of these then it is there for a legit purpose and can be left alone.
What is the file name and location of the HTML:RedirME-inf[trj] detection ?
Deletion isn't really a good first option (you have none left), 'first do no harm' don't delete, send virus to the chest (a protected area) and investigate.