A few comments if I may:
If a person is injured at the work place, I always suggest that they consult with an attorney who specializes in such matters. Initial consults are usually free of charge. Work comp laws can vary drastically from state to state and I would not allow my employer to be the sole source of information regarding my rights under the laws of my individual state.
I would be skeptical of any employer who tells an employee that they will not be covered for an injury incurred while lifting because they did not use lifting equipment furnished by that employer. Work comp is generally a no fault situation: In return for guaranteed/specified benefits for an injured worker, that worker can not sue the employer over and above those benefits. Are there exceptions? Of course, but that's the usual deal. The point is, even if the worker does something stupid (for ex removing the safety guard from a power tool) and that worker cuts off his hand, that worker is still covered. In most cases that worker CAN later be fired, however, for failing to follow company rules.
Now I don't know where the poster resides who believes that nurses have a right to no manual lift workplaces, but in the vast majority of states, nurses have no such right. Progress is being made, but even in some states with no manual lift laws on the books, the language/guarantees are so weak that most nurses still lift patients by hand.
Also, while using good posture and body mechanics is helpful, doing so will NOT prevent lifting injuries. The ample research clearly shows that without appropriate equipment it is simply not possible to prevent injuries.
Finally, with respect to the original question, the employer can specify on the job description whatever weight they choose for nurses IF that employer can show the essential functions of the job necessitates being able to lift that amount. If an employee can not perform those essential functions with or without reasonable accomodation, the employer is under no obligation to retain that employee.
The whole thing can be quite complex....union contracts, state laws, facility policies, federal laws all can have an effect on a given situation/case.
Which brings me back to my original recommendation....if weight lifting requirements are adversely affecting your employment status, or if you have been injured at work, see an attorney to learn your rights.