" The concept of a soul is an abstract non-scientific belief, so given its an abstract concept with no scientific basis, why do we make a distinction between one form of life and another?"
A 'soul' is a rational and scientific concept, it's just not materialist or rather, the kind of science we are used to daily.
As far as souls, more complicated, but maybe one simple answer is that more complex and intelligent forms of life have a deeper access to 'the soul' and that it's not like a step function, rather ... some kind of increasing gradient from inanimate matter, to plants, to animals, to humans, to the 'next level' of life.
My second answer relates to the first, i.e. 'Scientific' in sense that his is literally how we divide the universe at least biologically, i.e. life/not life, then domain, kingdom etc..
Please reconsider the notion that 'soul' has no scientific basis, as many would consider that the 'soul' or 'spirit' or 'animus' or whatever is literally the thing that distinguishes us from inanimate material - otherwise - according to a purely materialist perspective you cannot be alive i.e. there is no such thing as life - you are merely a random bag of particles that appears to be alive :).
If you believe that you are alive, that there is love, intelligence, happiness, experience, consciousness, and that you are not simply a purely random bag of particles, then it logically follows you must 'believe in something' that is not bound by strict materialist ideals. This metaphysical concept is not new, and we have some words for that, usually: 'soul' or 'animus' or 'spirit' or whatever. It's not irrational and it's not unscientific, it just by definition does not fit within materialist framework. And to be more clear - materialist science holds as a presupposition that 'everything is a random wave/particle/big of energy' - it is not a 'proof' kind of thing. It holds as a presupposition that 'there is nothing more'. In essence, materialist science starts with a premise of denying all other views, and then goes on to try to explain everything through the lego-blocks of matter type theories. Which don't explain life very well at all.
Sorry for all that. Short answer: Animals are on the spectrum of life, as are we, and there's clearly a relationship there and we can therefore infer a degree of their experience and status in all of this to some extent.