What are the logical underpinnings of this? Why should creating a suffering being have potential for endless negative utility, but the positive utility from said being feeling happy be capped at zero?
To the first question. I think to most people, it's an instinctual you believe it or you don't. But I'll try to rephrase in case that helps.
World A: There are 10 beings in existence. All super happy.
World B: There are 20 beings in existence. All super happy.
World C: There are 20 beings in existence. 10 super happy. 10 would rather have not been born.
World A and World B are equally good worlds. World C is a worse world.
Hence, if you are in World A. And have the decision to produce 10 more people. At best the world stays equally good. While there is the possibility for it to get worse.
To your second question. My earlier capping of the above situation at equally good was simplistic. There is also
World D: There are 20 beings in existence. All are super duper happy.
Which could happen if bringing 10 more beings into existence made the original 10 happier. So the cap on moral goodness, is how much additional happiness it brings to the people who already exist in the present.
World C is worse then World A by an amount equal to the net sadness of the 10 extra people. Hence, the cap on moral badness of bringing more people into existence. Is the net sadness of the immediate new people, and the net sadness of all new people that will arise in the future due to allowing the immediate new people now (like grandchildren). This probably isn't an infinite cap, given heat death of universe and possibilities of extinction happening anyway in the future (due to supernova or whatever). But the potential is still pretty large.