Tobacco is Tobacco, no matter what form it is in.

Smoking and chewing lead to health problems = liability.  Not to mention unsanitary.

Smoking can cause fires = liability.

Farming already has enough liabilities to contend with besides those that the hired help induces on themselves when being provided insurance. 

Besides, for every top-notch farm hand that smokes and chews, 20 that don't can easily be found.