1]
It is not a virus but ransomware.
2]
avast does detect many variants of TeslaCrypt.
3]
There is not tool that detects all malware.
4]
Many people have pup detection disabled in avast.
TeslaCrypt is (amongst other ways) spread through pup's.
5]
Detection can only be added if avast (and other malware vendors/developers) have a sample of the malware.
Doctors can't develop a cure for a decease that the don't know the existents of.
6]
I have to guess here, but those people are using a account with administrator rights for daily use.
That means that if malware gets on the system it has the same rights as the user.
NEVER use a account with administrator rights for daily use.
Security on/for a system starts with what the user knows/does, not with software.