Many industry horror stories abound with the problem of turning over interviewing to HR. In fact, there is at least one prominent book that explicitly states "don't turn over your interviewing and hiring to HR".

I keep second-guessing myself, but so far everyone is supporting my side of it.

I was personally attacked in the discussions and this felt like a clear power or authority assertion. I never did get a good reason why, just "YOU CANNOT!"

I've been asking for clarification and asked if engineers weren't allowed to interview engineers any more because that's strictly a non-engineering function. I was told I was an idiot and that wasn't what we discussed. So I guess the rule is that engineers will continue to interview engineers, but engineers are not allowed to discuss the interviewing of engineers with other engineers unless chaperoned.