Mostly it's to prevent brokers from giving customers kickbacks (either pure money or gifts). When a broker is helping people make decisions about what plans to buy, it seems like a reasonable restriction, although I don't think the intent was to prevent what Zenefits is doing.
Kickbacks just means "charging customers less money" and "giving customers a better deal than their competitors" and "competing in the free market".
The insurance industry is so messed up that they have made it illegal to give customers good deals and have made it illegal to compete.
Imagine that a bunch of companies got together and decided that everyone should increase their prices. In the normal world that would be called illegal monopolistic pricing. In the insurance world, charging people less money and giving consumers a good deal is the illegal thing.
Kickbacks (especially in the form of gifts) often go to the individuals rather than the company.
A broker who gives a 10% discount to a company saves the company money which allows them to get their employees a better plan (or higher profits or whatever).
A kickback of steak dinners and free golf for the HR manager is more like a bribe: no benefit accrues to the company itself.
Insurance prices are often approved by state regulators. It requires approvals and the state provides backstops and other safeguards. Prices in the insurance industry are pretty controlled. This even goes so far that despite there being a state backstop to many forms of insurance, a broker or agent aren't allowed to even mention this safety net without violating ethics laws. As for "why", I'm not completely sure. My guess here is it's to minimize the chance of the state needing to get involved with bailouts and to reduce competition so that companies don't take on unreasonable risk playing the odds that they'll make it through another year on the happy path without paying claims.