In general there is no legal or moral barrier to being paid for your silence, as long as you are not covering up an actual crime by doing so.
Keep in mind that this isn't an employment contract clause, it is a cash bribe to stay quiet. Your freedom is not impaired: you have the choice between saying whatever you like or taking the money.
No judgement on GH but isn't a cash bribe to cover up harassment exactly that - covering up a crime? California doesn't have state laws against harassment?
With the massive prevalence of twitter, facebook, tumblr and other social platforms where people not just speak about work but are actually hired in this day because of their crowd appeal, I think these agreements are a lot less about covering up crimes.
Far more torrid things have happened at the biggest companies in the world. Stuff like outright sexual favors for advancement. Just bad stuff, the big difference seems to be active HR, active corporate attorneys and the general ban on talking about what goes on at work in your social media. It doesn't take many 6 figure settlements to "fix the culture" at a place, and if fixing it means removing a highup officer, that happens too (http://www.bizjournals.com/washington/news/2012/11/13/lockhe...)
Honestly, I'd think more start-ups would start to develop more serious twitter and social media policies. I don't know of one that didn't have some disgruntled former employees, just due to the demands and the personal nature of start-ups.
Keep in mind that this isn't an employment contract clause, it is a cash bribe to stay quiet. Your freedom is not impaired: you have the choice between saying whatever you like or taking the money.