There are two principal reasons for this reluctance. First, some employers assume that the policy will bring about the unionization it is designed to avoid. Second, some fear for the legality of the policy. The first fear is entirely unfounded. Discussing unionization with employees is no more likely to bring about unionization than is sex education in high school to create impulses in teenagers that they don't have already.
All employees occasionally think about the potential benefits of unionization.
The second concern, though more understandable, is still misplaced. But the waters here are deceptively dangerous. The theory here is that the employer requires its employees to agree to remain union-free. Therefore, employees should not be required to acknowledge in writing that they will abide by the terms and conditions of the handbook. Instead, the certification they sign should be limited to acknowledging receipt of the handbook with a pledge to return it, making no copies, upon termination of employment.