Are you suggesting they allow spammers? Every time I remove a spammer post I submit it to /r/ReportTheSpammers, and they get killed almost immediately. What else can they do outside of that? Not allow new users to vote and comments?
The best they can do is make spammers be unclear on if they're having any effect or not, by obscuring votes, or by secretly banning them without them realizing.
It's a tough problem to get right, especially without making the experience worse for everybody else. I think they've found a happy medium.
This is the entire point.
That said, I wish they'd just ban spammers and give everyone accurate metrics.