Greg, as you noted, such restrictions are so unusual that they need to be specifically called out in advance. Likewise, such agreements are subject to a great deal of litigation ... and frequently not enforced by the courts.
Not that it has matters .... the outlawing of slavery did not prevent Texan employers from foisting 99 year 'contracts' on illiterate former slaves, and attempting to have them enforced.
None of that applies here, for two reasons: there is no such specific agreement, and none of the parties are employees.
In the example you cite, I suspect that IBM relied upon the idea of 'at will' employment. That is, essentially, what the GC is threatening to exercise: do things my way, or I won't let you play with my ball.
Otherwise, all cant aside, let's look at the practical barriers to enforcing the 'understanding' that the GC is asserting exists:
Assume the customer has an additional project. Is the first GC asserting that the customer must hire him again?
Assume a job has been bid by several GC's. Is our GC asserting that the sub cannot furnish bids for his part of the job to several GC's?
Assume a job arises that does not require a GC. Is the GC asserting that the sub is required to hire him anyway?
Assume the customer - or anyone, for that matter- has obtained the sub's number from the truck, the phone book, another referral, etc. Is the GC asserting that the sub is barred from accepting such work?
Suppose the GC needs another sub for some task. Is the heating guy supposed to stop taking service calls because he did a job with the GC? Or the plumber? Would the GC be upset if the roofer returns after a storm to make repairs five years from now?
I'll be that, should there be an accident on site, that the GC suddenly becomes an expert on the topic of 'independent contractors.' He's not about to let that loss affect HIS insurance premiums! Or pay the sub's unemployment insurance. Etc.
The GC can't have it both ways. Either he has employees, or he hires a contractor. What happens apart from that specific job is of no concern to the GC. He surely would object if the sub insisted that the GC only use him .... or only bid on jobs with the sub's approval.
Ironically, for the GC, the electrical contractor is in a much stronger position; in some ways, it is the EC who's in charge - not the GC. Unlike nearly every other contractor, the EC pulls his own permit, and signs off on the plans. He goes, the permit is void.
Contract law is based upon the 'legal fiction' of the parties being equal. Maybe it's best to make that fiction a reality, and leave all the other baggage aside. Call it the 'four corners' doctrine: if it's not in the 'four corners' of the contract paper, it's simply not part of the agreement.
