Thanks for the suggestion.
The short answer is that they're essentially ignored. The backup preference is only a very small portion of the algorithm that VEEAM uses when selecting which node to backup transaction log from. In most cases, whichever node finishes the nightly (image) backup first will end up being the primary.
The reason I opened this was because VEEAM Support confirmed there wasn't a way to force it to use the primary. If you look at the Case#, you should see that QA was consulted in the behavior.