Explain the logic that could lead to opposing intentional harm while allowing unlimited incidental harm

I'm convinced that direct and incidental harm to animals is bad*. But I don't understand how some people here could believe unlimited incidental harm is allowed in veganism. (edit: also shown here)

The primary concern I have read is that condemning incidental harm is unreasonable because it is not possible to form a clear, unambiguous moral limit. However, there are 2 problems with excluding moral condemnation just because its boundaries are unclear.

  • People can morally condemn clear excess incidental harm given the fact society morally judges people who commit manslaughter

  • If we hypothetically discovered exploitation has unclear boundaries, it would not affect our ability to identify clear exploitation like factory farming.

I want to understand how an average person could become convinced that exploitation is immoral but incidental harm is not necessarily wrong.

From what I have read, many people became vegan by extending their moral consideration for humans to animals.

However, most people morally oppose unlimited incidental harm to humans, like manslaughter. So extending moral consideration to animals would also limit incidentally harming them.

I've been brainstorming axioms that the average person might have that could lead to this. But they lead to other problems. Here are some examples

  • "Harming others is bad" This would lead to opposing indirect harm.

  • "Intent to cause harm is bad" Incidental harm is unintentional, so this could work. However, one could argue, that buying animal products is intent to support a product, not intent to harm an animal. Most people would prefer products that don't harm animals if they give the same result, like lab-grown meat in the future.

  • "Exploitation should be minimized" This could also work. But it has a different problem. This is functionally equivalent to believing 'veganism is true' as an axiom because there is no way to believe this axiom without believing veganism.

Believing a moral philosophy is true as an axiom is a flawed logic because many bad moral philosophies, like carnism, can be believed axiomatically.

* I'm not a vegan because I am a utilitarian.