The distinction is based on methodology. Ideas about natural selection acting on randomly occurring mutations can be tested by examining living organisms, observing existing ecosystems, and studying the fossilized remains of ancient organisms. In other words, this restriction of evolution then allows it focus on the concrete evidence that is available.
Abiogenesis, on the other hand, deals with an event for which there may never be any actual evidence to hold in our hands. The best that will ever be possible is to use geology to determine the conditions on the ancient earth, try to replicate portions of this understanding in a laboratory, and see if this sheds light on an event for which we may never have direct evidence.
The other reason for the split is that the two concepts are logically independent of one another. One can have no idea of the origins of something but have ample evidence for the subsequent history of that something. My own family history is one example of this; we have no idea how my ancestors first came to Ohio, but we have a clear understanding (backed up by the physical evidence in the form of birth/marriage/death records) of how they travelled from Ohio to Kansas to Oregon, up to my own birth in Alaska.
I think this is the main reason for the distinction. We have a pretty good record of the history of life on this planet (at least from a certain point), but we have no record at all of its origin, and, until recently, not even any good ideas about.
But as you say, the distinction is probably arbitrary and artificial. But, then, all distinctions and categories are artificial. We divide the world into categories to help us make sense of it. In this case, we divide evolution from abiogenesis because it is useful to distinguish between something for which we have ample evidence, and something for which we are still trying to understand even the most basic elements.
"The fact that a believer is happier than a skeptic is no more to the point than the fact that a drunken man is happier than a sober one." -- George Bernard Shaw