If an employer pays for and allows respirators to be worn on a voluntary basis when respiratory protection is not required to meet any OSHA standard, is it then regarded as though the employer is mandating the use of the respirators?

No, the employer may allow the voluntary use of respirators even where an exposure assessment shows respirator use is not required. The use of respirators is not regarded as mandatory unless required under the standard and/or the employer requires that employees wear respirators regardless of the exposure assessment results. Note that if you permit the use of respirators other than filtering facepieces, you must pay for required medical evaluations for voluntary users and provide voluntary users with appropriate facilities and time to clean, disinfect, maintain, and store respirators.