No one's saying they can't be allowed such a method. But it may be the case that for persistent business relationships doing so may mean an employee/employer relationship rather than the relationship between a business and an independent contractor.

I believe the distinction lies in describing the final work product vs. describing means and methods. If you have an outsourcing contract with an independent contractor, you're allowed to specify (usually via specifications that are appended to the contract) performance objectives and quality of the work, and what the final work-product looks like. If you stray too far or too often over into specifying the means and methods by which they're supposed to achieve those results, then you're treating the contractors like employees, which actually come to think of it, is already sort of a no-no under most current contracting arrangements I'm aware of. Although there are always gray areas... for example what if you're turning over source code at the end of a project? Then your writing of the code is both a method and a final work-product. Some attorney needs to earn their keep and make sure everything is all laid out nice and kosher in the contract so things don't get murky.

A scoring system of the resulting work is fine. A scoring system that effectively encodes how they did the work is problematic because it means they company is now dictating how work be done, which is an employee relationship.

E.g. It's the difference between: a scoring system that uses a coding style standard and unit tests on the resulting code to build a score; and a scoring system that uses hours logged into the computer manipulating the IDE and lines of code written per hour to build a score (the contractor could use that as a way to score himself and for billing, but not the company).