Many companies have policies which prohibit this. It's not even specific to SV or the USA.
I'd see it as protection of both parties. The company doesn't want any reputational damage or off-message communications. The employee doesn't want to fall afoul of any liabilities they might place upon themselves or the company by revealing or promising anything they shouldn't. So personal social media activity is not prohibited, but social media activity in the context of your employment by the company is.
I suspect Apple might be far harsher than most companies if people step over the line, based upon previous examples.
I'd see it as protection of both parties. The company doesn't want any reputational damage or off-message communications. The employee doesn't want to fall afoul of any liabilities they might place upon themselves or the company by revealing or promising anything they shouldn't. So personal social media activity is not prohibited, but social media activity in the context of your employment by the company is.
I suspect Apple might be far harsher than most companies if people step over the line, based upon previous examples.