Because there is still an off-hand chance that an employee who has been let go isn't speaking out of spite and merely stating the facts - depends on a combination of their honesty and the feeling they harbor about being let go. Everyone who is let go isn't bitter and/or a liar.

However, every company yes-man is paid to be a yes-man and will speak in favor of the company without exception - that literally is the job. Otherwise they will be fired and will join the ranks of the aforementioned people.

So logically it makes more sense for me to believe the former more than the latter. The two-sides are not equivalent (as you may have alluded) in term of trustworthiness.