PCI database updating using with DNS resolv support need to support with other USE-flag than "dns" due to clash with standard meaning of "DNS". Because in other packages (such as net-analyzer/monitoring-plugins or net-proxy/tsocks) it means that utility support DNS. But in pciutils "dns" USE-flag means that DB must be synchronized using DNS than other way. It has other semantics.
The package has an appropriate USE description: Enable support for querying the central database of PCI IDs using DNS