It'd be for the same reason people aren't trusted to moderate their own topics. You just can't trust people to be rational about which comments they remove or keep. Comments are there to give the browser an idea of what they're about to download - if the submitter removes all the negative comments it renders the feature fairly useless.
How about a way for the mod submitter to "flag" comments on their mods, not entirely delete them, and people have the option to show flagged posts or keep them hidden?
It's less dictatorial than deleting comments, but doesn't require huge amounts of babysitting by RTB admins. People can see when developers are minimizing spam versus censoring opinions.
However, it would probably be more work to implement. Anyway, that's my two cents.
