The answer to why an employer would dispute a workers' comp claim is simple and boils down to a single five letter word: money. Like any other type of insurance product, employers pay premiums to provide workers' compensation benefits to workers (in most states, this is mandatory). Premium amounts are directly affected when injured workers file for benefits.
Logically, the more workers' comp claims that are filed, the higher the costs for employers. Workers' comp insurance premiums increase when more workers than estimated file for claims, or when an employee has a particularly expensive claim (for instance, requiring back surgery). It is for this reason that employers and their insurance companies routinely use investigative agencies to monitor the daily activities of workers who have filed workers' compensation claims.
Unfortunately, many employers don't believe that some injuries are serious or even valid, especially cumulative trauma injuries. They assume that a worker who files for workers' compensation benefits on the basis of carpal tunnel syndrome, another repetitive stress injury, or a lumbar back injury is not being completely truthful (or is "malingering," the industry term for feigning sickness or disability for financial gain).
Employer bias is particularly strong against injuries involving inexplicable pain that cannot be wholly verified by medical examination, or even sufficiently verified by x-rays, other imaging, or nerve conduction studies. Does this mean that the injured worker who has constant back pain is malingering? Definitely not. Many medical conditions are difficult to objectively verify.
If your employer or its insurance company denies your claim, or any part of it, it should inform you in writing. Typical reasons given for denying a claim are:
If you receive a notice that your claim has been denied, call or write to your employer's workers' comp insurance carrier. If this doesn't solve the problem, hire a workers' comp lawyer and request a hearing with the state workers' comp board.
The bottom line is this: employees who have become injured or sick as a result of their job should file for workers' comp to protect themselves, and if their claim is denied, they should fight the insurance company, with the help of a lawyer. Whether or not the employer believes that the worker is legitimately injured will turn out to be irrelevant, and the worker shouldn't worry about whether the employer holds the employee in contempt for filing a claim. It is the worker's right to have time off work and medical treatment paid for; the worker has given up the right to sue the employer in exchange for the workers' comp benefits and should not feel guilty about using them.