Employers can spend quite a bit of money keeping the employees outfitted in their planos. Of course, the employer is still liable if there is an OSHA audit. PPE must be worn where there is danger of injury. Therefore, most employers elect to pay for the eyewear in order to enforce compliance. It is thought that nice-looking, more expensive planos tend to stay in the employee's possession longer and that they take better care of them.
Some employers even pay for 'an extra pair for home' right up front.
Prescription frames are considered personal in nature. Employers are not required to pay for them. Again, they often choose to pay for the eyewear in order to maintain control. And, of course, prescription safety frames are often part of a contract negotiation when there is union representation. It is important that scratched or nicked lenses be replaced immediately.