1. Contractual obligation: When someone signs a contract with an employer, it is considered a legally binding agreement, and if it contains clauses specifying the consequences of breaking the contract, then it must be followed.
2. Compensation for damages: If an employer has incurred losses in the event that the candidate backs out from joining the company, then the employer is entitled to receive compensation for the damages caused.
3. Deterrent for candidates: The imposition of a penalty for leaving a job offer before joining can discourage candidates who have no intention to join the job from accepting the offer.
4. Fairness: Companies invest time, effort, and resources on recruiting candidates, and should not be at a disadvantage from backing out for their reasons. It would be unfair to the employer if this was permitted.
5. Context: Depending on the circumstances, such as the size of the company or the scarcity of a particular skill set, the amount of the fee can be negotiated. However, in instances where the employer specifies a significant financial penalty, it is reasonable to assume that they're protecting their interests.