The WAC is a "rule", written by someone in DFW, without a law(other than time/place/and manner) to back it up. The issue would be proving a person was "hunting" deer with a rifle during the archery only season. Transporting a rifle (with a valid bear tag) is not per se illegal. For clarity sake, one might keep the rifle unloaded and shells in a backpack until actually hunting bear. Or carry a large caliber handgun and use it for bear as well as personal protection.
WAC is Washington Administrative Code codifies regulations and is enforceable law. All WAC codes references the statutory authority upon which it is based.
If you want to rifle bear hunt and archery deer hunt, you cannot legally have both weapons on your person during archery deer season. Your only legal option would be to hunt deer during rifle season with a bow.
The reference for this instance is Revised Code of Washington (RCW) 77.04.055. The legislature gives WDFW the authority to define what gear may be used to take wildlife. In the WAC, they did this, thus it is enforceable law.
RCW 77.04.055
Commission — Duties.
(1) In establishing policies to preserve, protect, and perpetuate wildlife, fish, and wildlife and fish habitat, the commission shall meet annually with the governor to:
(a) Review and prescribe basic goals and objectives related to those policies; and
(b) Review the performance of the department in implementing fish and wildlife policies.
The commission shall maximize fishing, hunting, and outdoor recreational opportunities compatible with healthy and diverse fish and wildlife populations.
(2) The commission shall establish hunting, trapping, and fishing seasons and prescribe the time, place, manner, and methods that may be used to harvest or enjoy game fish and wildlife.