When an employer sends a salesman out to sell his product, he is sending him out on the company dime to wine and dine customers and get them to sign that contract for services rendered or product delivered. This costs the employer a great deal of money. The salesperson is worth nothing except for the relationships he can create between the employer and the client. These business relationships are created on behalf of the owner, not the salesperson.
So when a salesperson leaves a company and then decides to use the information he gathered during his tenure there to create new business elsewhere, he is essentially stealing what he was hired to create in the first place. I don't know of many companies that allow their employees to walk off with proprietary information, and information related to core business functions is certainly proprietary.
"Create a better product" doesn't fly. Anywhere. Not even in California. They simply call the list of customers a "trade secret" and sue your ass when you decide to start dialing them up at your fancy new employer.