Part of it is, if anything can be taken slightly out of context to imply something discriminatory, there are those who will abuse the system and sue. At a large enough scale this can become a real problem. If the company policy is "never say anything" there's nothing to be taken out of context, reducing the chance of a lawsuit.

I bet you this comes back to insurance, as many things do in the corporate world. Sufficiently large companies probably have insurance coverage for discrimination lawsuits, or at least employment disputes in general. The coverage probably costs less if you have a "no feedback" policy.