From what I've seen, parties who object to animal cruelty/mishandling don't cite an ounce of logical proof behind their stance. It's as though they believe that their sentiment stems directly from common sense and as such does not necessitate a formal explanation. I understand that this is often a defining characteristic of interest groups, but it seems unusual for one with such great traction in modern western society.
Nearly all of these activism organizations rely on shock and gore media to garner support, taken to extremes by the more popularized instances (e.g. PeTA). At the end of the day, their sole plea is to eliminate suffering inflicted upon animals...for no other reason than because it is within humanity's capability to do so. I've scoured through a dozen sites in the past half-hour, and this generally holds true to each one.
The only legitimate thesis I could find essentially claims that animal abusers are liable to harm humans due to moral desensitization, which actually makes a fair amount of sense, until you realize that "abuser" might simply be referring to the common pet-owner or whatnot. Do you guys know any others?
TBH, your post comes across as rather a bit...robotic, to put it generously.
In any event, I have a paper being forthcoming in the Columbia Journal of Environmental Law in a couple weeks on animal rights under Jewish and Islamic Law as compared to American law. Too bad it's not published yet, or I could plug myself here lol
To understand why many people think the same re. animals, perhaps trying a different starting point might help: why single out humans? There's fairly good arguments (https://www.animal-ethics.org/the-idea-that-only-humans-are-... and the linked papers are a good resource) to be made that many other species have a similar capability to have positive and negative experience (sentience) which are not just citing 'common sense' or similar weasel-words, instead actually looking into the neuroscience of pain and other suffering.
The "human dignity is damaged by mistreating animals" argument isn't a bad one, as you note: people who cannot empathize with animals are more likely to fail at empathizing with other humans.
Like all agenda driven by emotion and virtue signaling, you can reject with a cursory look at the matter. Does that mean we should allow unnecessary cruel behavior towards animals? No. We should be the best stewards of animals, plants, etc on earth we can be.