You could say that about a lot of things that are normally covered by abuse policies. For example, we expect hosts to remove content that violate copyright, why is copyright more worthy of protection at the corporate level than defamation?
Because society created laws in that way?
The DMCA specifically requires hosts to remove content upon an allegation of copyright infringement. There is no law requiring hosts to remove defamatory content (without a specific court order). Congress has deemed copyright protection more worthy of protection at the corporate level than defamation. If you are unhappy with that situation then you should contact your members of Congress.