Legal Question in Business Law in California

Well, first, if you are truly a consultant, it's arguable that you are already running your own business -- a consulting business. The problem is that the term "consultant" is used more and more these days to describe employment and quasi-employment situations where the employer and perhaps the employee/consultant as well are simply trying to avoid the baggage of the employer-employee relationship, including workers' comp.

Whether the clause in question has any real legal weight will perhaps depend upon your true status with the offeror after you accept. If you are legally a consultant, the contract clause might perhaps be enforceable. If you are really legally an employee in consultant disguise, you'll have the rights given employees under the Business and Professions Code 16600 and the provision would be unenforceable and void.

A close reading of B&P 16600 might suggest that a contract provision restraining even a consultant from servicing other accounts might be void, but I think as the statute is construed and enforced you are much safer if you are an employee than if you are a bona fide "outside" consultant who has contracted to provide full-time services to this client.

Finally, I should mention that you may be in a position to negotiate for the removal of this contract provision, especially if you are going to be a true independent outside consultant as opposed to an employee in consultant disguise.

What could make you think it doesn't 'preclude'???

It is obvious that the intent of the agreement is to obtain exclusive use of YOU full time, and to prevent 'competition' and conflicts. If you don't like the terms don't sign. Violate the contract terms and you'll be in breach. Read it to see what their damage claims will be for breach. Whether the terms are ultimately enforceable or not, you could get sued for breach and have to defend the lawsuit.