That's interesting. As a contractor I saw them whenever subcontracting, but they were always willing to clarify to the point that they didn't want me contracting with any of their customers directly (and usually only ones that I met through the parent contractor).

As an employee, every company I've ever worked for has given me one. Few have been negotiable, but pretty much all have openly stated/agreed they weren't enforceable in CA when I raised concerns.