As I understand, sandwich leave is the employer's practice of counting the holiday as leave if it is preceded or followed by any leave applied for by the employee. Whether such a leave policy is right or wrong is a different question, as any answer in this regard would be a nuanced one.
Purpose of the Sandwich Leave Policy
The main purpose of such a policy is to discourage employees from availing a longer period of authorized absence inclusive of the weekly holidays in the interest of the organization's work both at the beginning and the end of the week. The negative impact of such a policy is that it curtails the right of the employee to combine their usual holidays with the leave applied for, even on occasions of genuine urgency. Thus, it is just a preventive check.