Why do Sales people have notice periods?

Why do Salespeople have notice periods?

A friend of mine works in Sales, and he recently handed in his notice as he found a job with a competitor. There is a notice period in his employment contract of 4 weeks. But because he is in sales, the company ALWAYS walks the employees out the door when they resign.

They NEVER work the notice period.

But because it’s in the contract, they have to pay four weeks salary to the person leaving, which brings me to the obvious question. As an employer, why would you put a notice period in the employee contract for your salespeople?

Notice periods should be there to protect workers if they are suddenly let go or made redundant. But if it is the employee’s choice (they find a new job), why would an employer insist on a notice period they never intend to enforce?

From asking around with my other friends and colleagues in Sales, it seems that this is the norm when it comes to their employment contracts (not a statistically valid sample, I know, but still interesting).

Does this happen intentionally? Or is it a result of the way we always do things?