No one is "reducing" anything.
Altman is not justifying the arbitrary bans (or any Reddit admin shenanigans) here, that is not the topic of this sentence.
He is talking strictly about one dangerous activity that gets offenders banned: death threats, which Ellen Pao (the human at the other end) received.
We don't know why Pao went out. We can suspect it was the backlash (more likely), or disagreements with the board [0] but we can suspect.
But that does nothing to make her opposition entirely legitimate, when at its strongest it was hijacked by namecalling, cyber-bullying, and yes, threats to Pao's life. The subset of Redditors who did this deserve to be told how shitty it is. The remaining, legitimate opposition should recognize the distinction.
[0] - http://www.nytimes.com/2015/07/11/technology/ellen-pao-reddi...