They "sponsor the work" because the work is already done. Apple would only be creating more headache for themselves down the line trying to refactor FreeBSD patches that don't include their upstream code. By paying other developers to publish their work, they effectively launder the responsibility of maintaining to their code to the community.

It's a pittance. Like complimenting Google for all their "generous" contributions to the Chromium project even though Chrome is used as an accessory for the AdSense monopoly.