Because 4chan's ads aren't invasive and they don't track you. They're just small banner images displayed at the top and bottom of the page with no JS, no flash, etc.
Precisely the type of ads we should be celebrating since they subsidize the content we consume. A lot of people either are oblivious to their actual importance, or just don't care. The internet simply wouldn't be what it is today without them. True, there has been plenty of bad (trackers, malware, etc), but the good that has come from that subsidization has lead us to where we are today. A vast majority of the innovation on the internet has been directly or indirectly related to that subsidization.
I can determine what ads I do and do not want to block. How many other ads does Focus by Firefox not block?
Edit: Y'all downvote weird shit sometimes. Marco quit the ad blocking game because he didn't want to be the arbiter of what did and did not get blocked. It's perfectly reasonable to question what a given blocker will and will not block. And perfectly valid to wish to have that control.
Also, on a more technical level 4chan's ads aren't served from a well known ad host like Google, so they wouldn't be blocked by a typical blacklist without filtering URLs containing ad -- which some adblockers do, causing [all kinds of problems] [0].
