[Stockland Hillman]

Wrong, I'm afraid in every respect 

Companies and suppliers usually have contracts which cover all issues of conduct where the conduct may affect the position or reputation of the contracting party. 

It's a basic tenet of business,  used widely where outsourcing or franchising takes place to protect the master organisations reputation.  They also cover post engagement behavior

In law it's actually easier to control a contractor than a staffer

Wrong, for the simple reason that you’re confusing contractual provisions on reputational damage with day-to-day control. The former doesn’t touch IR-35. The second very much.

The BBC argues he breached impartiality rules. He is, as long as he is not on air or making comments on behalf of the BBC, not subject to those rules. Just as Andrew Neill and Chris Packham were/are not.

But I’ll leave that there as it would derail the thread.

Simply not true.  You're conflating a commercial contract with staff rules 

A contract can require anything if both parties agree on signing. What's said, done, auctioned.  The parties would just in advance agree scope and limitations.

Like I said,  it's easier to control contractors than staff , for lots of legal reasons 

IR35, staff social media guidelines,  public statements etc are all meaningless in this situation.  The only thing that matters is the GL & BBC contract terms