Is such an idea legitimately or just another cherry-picking around to justify personal incapacities and giving favors?
Perhaps it's because the suttas and/or the precepts say e.g. "don't kill" but don't have as much detail as the Vinaya.
For example, if someone asks, "What if I cause someone to die by accident? What if I dig a pit and someone falls into it?", that's maybe not a question which is answered explicitly in the suttas or precepts -- but is a question that's considered in detail in the Vinaya.
So I suppose some people would prefer to rely on what they know of the Vinaya, instead of rely on their own interpretation of the suttas or precepts.
You might want people to rely on their own experience too (not just theoretical doctrine). But I suppose most people have little or no personal experience of digging pits and seeing other people die in them, so an answer based on theory might seem like all that's available.
It's not just "killing" -- I think the Vinaya details the necessary components of other offences (e.g. lying), which look like lay precepts too (e.g. the fourth precept), including ...
- Intent to deceive
- Effort to produce the deception
... which isn't detailed in the suttas (though you might justify it from the suttas). That's a topic which probably affects everyone (more than pit-digging does).
People do ask that kind of question -- see e.g. Trolley problem and its variants in Buddhist ethics (whether they should ask is another matter). Here was your answer to that, by the way:
It may lighten or make certain speculations irrelevant to be clear that one does not have any inherent obligation to "help" or "save", but this it self, the notion, is in fact an "obligation" out of unskillful gained debt, independency.
When I read it I assumed that Suminda's answer was based on the Vinaya (because its content is so similar). Reading its references now, though, I see it's based on a Tharavada text called the Atthasālinī, a commentary by Buddhaghosa on the Dhammasangani -- which ties somehow to what you said in the OP about Abhidhamma. I don't know from where or how Buddhaghosa derived those doctrinal details.
The "precise" definitions sometimes have behavioural consequences -- e.g. (rightly or wrongly) some people don't classify copyright as stealing, because making a copy doesn't deprive the owner of the original thing and/or other reasons (not that I want to debate that again here).
Incidentally I think you sometimes use "killing" and "stealing" metaphorically, which is hard to converse with -- "When will you stop beating your wife?" is an example of a so-called loaded question (actually I find a lot of your language is "loaded" in other ways, e.g. your saying "draw excuses" in the this OP above instead of "give reason" or "explain" has me thinking that you condemn that practice, and assume that you know better -- I guess the "law of nature" which you refer to later is "dhamma").
One more thing, I find that suttas don't explain karma in every detail. Perhaps I see in general what karma is, and perhaps what it isn't -- but I think that "the precise working out of karma" is one of the Buddha's supernatural powers -- I'm not sure I could answer any question about the "weight" of karma, though people try to compare different weights (see e.g. ruben's answer), with justification from the suttas.