[matz1]

Maybe arbitrary is not the accurate term.

I mean if the concern if experiments on live humans then why can't one just specifically address the concern and draw the line ?

[brigandish]

You could but it would indeed be arbitrary.

[matz1]

In that case every rule/law is arbitrary.

[brigandish]

No, because not every rule/law is a case of special pleading.

If you think it's not arbitrary then you are free to make that case, and then I or someone else will point out flaws like the special pleading that is already glaring. Peter Singer's work on speciesism is a good place to find out the obstacles you will have in constructing a non-fallacious, non-arbitrary argument.