I wouldn't say that PUPs are nuisances... or "easy malware" (so that AVs "don't bother"); it's something different.
PUPs are tools that some people may install on purpose on their system (and therefore don't want to detect them), but on the other hand - if such a file appears on your system without your knowledge, it's quite likely that this tool is being abused by some malware (or even a human attacker) - and something bad is going on.
For example: keyloggers, remote control tools, network sniffers, FTP servers, ...
So, it's not really that AVs don't care about detecting the tools - the question is whether the user actually wants these tools to be detected, because they are not bad per se, only if abused. (So, I believe the detection is disabled in avast! by default).