I have never agreed with the addition of "mechanicals" to UO in the first place. But that's a discussion for a different thread. Given that they are here though, by their very nature, I agree that barding should not affect them.
And even I though fully understand (and agree with) why the devs feel that some high-end non-mechanical creatures should be immune to the bard skill set I don't think that its ever been appropriately explained in an in-game context.
For instance the ronin (and also ninja) mentioned in the original post. Are they non-bardable because they're human? That won't quite work because there are other humans in-game that can be barded. Are they too smart to be influenced by bards? Again, there are creatures in-game that lore much more intelligent than they do.
I know I would have a much easier time accepting why my bard is nerfed if I could hear a reasonable in-game explanation.