Why is it unacceptable for humans and yet okay for animals? Christians would say it's because humans have a soul and animals don't. (They used the same argument in slavery days about folks of African origin.) But Buddhists don't believe that, do we? So what is the Buddhist rationalization?
Vegans are those who cannot rationalize away the slavery of animals. It doesn't help to say that they are treated humanely. That doesn't address the fundamental issue.
Om mani padme hum
A friend of mine would often tell me that life is cheep. After being reminded so much of this- and apparently taking this as an authoritative statement that required no further explanation - it got me thinking, Is life cheap, and if so why? Could it be so that many people have a strong sense of self importance and take themselves too seriously? In order to be so important we would have to make some sort of statement about this. Take for example the pharaoh's and their tombs, or the grand estates of monarchs and celebrities. How else can we be above all? By our form, our intellect, our gender, our race, our religion; anything at all really for someone insecure about their own sentience.
Do the pitfalls of insecurity actually lead to a cheapening of life? This would barely cover the issues raised if there is any fit at all to the questions you pose. But I do recognize you potentially raise awareness of double standards and cognitive dissonance. To be socially and culturally normative in modern life entails a degree of unquestioning consent. it isn't entirely down to laziness that we delegate tasks and issues to others, and it isn't always in our best interests to do so. At some point we have to connect the dots looking beyond the bubbles of singular existence, and maybe question the orthodox paradigm that means a larger slice of pie for everyone, in spite of an increasing demand for said pie.
It just seems to be so grossly inefficient and neglectful right now. Measures to cheapen and thereby bolster the effectiveness of approved systems is leading to other sorts of chaos such as antibiotic resistance and skyrocketing healthcare epidemics. Despite select sentient being so cheap and unimportant we do tend to use an awful lot of resources in unsustainable upkeep costs. Again, the general consuming public don't see this, and don't connect the dots. The effects tend to manifest themselves in an indirect way so it's not really enough for everyone to have a quick sit down and think.
Sometimes these thing just play out and the change become apparent of their own accord. Is karma a 'playing out' or unfolding? Perhaps I'm mistaken and waffle on too much.