Apple prohibits developers from listing details of what they're working on on their linkedin profile for trade secret reasons.
Countless companies prohibit salespeople from connecting to potential leads on linkedin to prevent it leaking to competitors.
I'm guessing you've not worked in enterprise because it's pretty normal to have a company policy on "bring-your-own-device" (typically companies will only allow access from devices that meet security requirements on password, anti-virus, etc. often they'll also require the ability to remotely wipe your device)
Every (serious) company prohibits it's employees from disclosing secret information, be it Linkedin or by phone, and even verbally with your friends. That doesn't mean it's forbidden to use Linkedin, make phone calls, or talk to people.