Why would your employer care what you invested in? How would they be impacted if you bought or sold public stock of someone they interact with? Would they care if you traded Google, Apple, Microsoft or Oracle?
One possible explanation is that we require this of our vendors and suppliers, and the easiest way to achieve that is by making it reciprocal.
The actual policy wording:
> No Third-Party Trading or Tipping. Do not trade in the securities of another company when aware of material nonpublic information about that company in connection with your work at ******. This includes trading in the stock of ****** suppliers, manufacturers, vendors, or customers, such as cellular network carriers or other channel partners. You must also not tip material nonpublic information about another company.
I think most employers wouldn't care at all.