Should Human Resource professionals be knowledgable of all the HR-related laws?
This depends a great deal on the organization and how the human resources department functions. Human resources encompasses an incredible number of laws and regulations, at the federal, state, county, and municipal level, and unless a human resources person is an attorney specializing in employment and labor law, this could very well be an unreasonable expectation. Just to name a few areas of law that affect human resources, we have law on worker's compensation, unemployment compensation, wage and hour, discrimination, family and medical leave, often, at the very least, in federal and state statutes.
In a fairly large human resources department, there is a tendency to assign someone to a particular area, for example, all worker's compensation claims or all unemployment claims. Then the person assigned to handle this area gains a great deal of knowledge and experience, better able to serve the company. Some organizations contract out for this work, to a law firm, or a human resources contractor. In a small company, a few people are responsible for all of it, but there is sometimes a lawyer on retainer to offer help.
Certainly, a person who wishes to enter the field of human resources has a responsibility to acquaint him or herself with the laws that affect human resources, but a true, in-depth knowledge comes only with experience, and few people in human resources have experience in every area of law.