The important element that defines the employer-employee relationship is the supervision by the employer. Supervision does not mean instructions to do work and similar routine supervision, but taking disciplinary action against the contract workers, fixing their wages, etc. In these circumstances, it can be easily established that the principal employer has knowledge about the employment of a particular worker over a period of time.
On the other hand, in a genuine contract, the principal employer only knows the number of workers engaged (not employed) and has no knowledge as to who all are deployed by the contractor. For the principal employer, it is the service that they want to get, and in delivering the same, whoever is engaged by the contractor is immaterial.
At the same time, many contracts are made to avoid the complications of collective bargaining. There is a general feeling that if work is done through contract labor, the employer has no legal obligation, whereas legally, they are bound to ensure minimum wages, statutory contributions, and the welfare of those engaged. It is true that the provisions of standing orders may not be directly applicable, but to what extent these would benefit the employer is minimum.
It is also doubtful if these workers would take the same responsibility that the regular workers take in discharging their duties. Moreover, such engagements are only possible in areas of operations that are not directly related to the main objective of the organization. The Contract Labour (Regulation and Abolition) Act has been legislated with the view of reducing this kind of employment. Needless to say, this Act remains unenforced in many areas of India, which are industrially very advanced! You may read this comment in the context of the Maruti incident!
Madhu.T.K