Well that flies in face of most every system in the world. The lack of contamination must be proven not the other way round. If true it is a very strange aspect of the legal culture.
I don't think it's strange at all.
I don't think it's even true that this would go against most systems in the world; btw most systems in the world are civil law systems, the UK amd US common law based and plain adversarial systems are a small minority.
Scientific evidence does not have a legal nature itself that makes inherently different from any othe piece of evidence. No piece of evidence is required to have a standard of certainity, or to be perfect, in order to exist. If it has a lesser quality, lacks an absolute certain and universally validated, it will still be given a value and a weight, comparable to its statistical value as proof.
And what is central to the case? This should be interesting.
This case is based on a large set of circumstantial evidence, like most cases, evidence which - as stated by the Supreme Court - must be assessed as a whole.
This evidence consistes in very many pieces of several nature and from multiple sources, which form a "system" (or a number of systems) amongst themseles, and they can be regrouped in several areas. I list here some of them:
1. the immediate phisical evidence of the scene, which is evidence pointing to multiple perpetrators (shows two different modus operandi/ behaviours of perpetrators); it is a system of elements roughly visible prima facie that includes evidence that some perpetrator returned to the murder scene some time after the murder, and also that spent some time in the murder room.
2. physical evidence pointing to the fact that the break in was staged; this is also a made of a set of physical things, which would distinguish this "break in" from the true "break ins" as they are always seen.
3. the autopsy report, crossed with other physical findngs. Despite the claims of the contrary, and despite the quotings from pathologists who were not sure about this based on their information, the combination peculiar set of injuries is evidence that Meredith Kercher was killed by more than one person; this is also crossed with contextual findings (like DNA on her wirst etc.). This evidence is absolutely important in my view.
4. Capezzali and Monacchia's testimonies: these testimonies are credible and related to the murder, no logical argument exists against this; these testimonies are evidence not just about the time of death but because they can be "crossed" with other areas (such as the autopsy report).
5. The luminol prints. This evidence carries a strong probative value, despite denials of those who try to dismiss it (TMB etc), because of its extreme peculiarity, because of its striking analogy with the bloody bathmat print, because of its correspondence with the peculiar activity of shuffling on a mat/towel after washing oneself in the bathroom; above all, because it's value as a contrario evidence, that is because of the absolute weakness of the contrary scenario, the lack of alternatives (no alternative substance, no alternative activity);
6. The lies of Amanda Knox (the mop-shower story). This topic is disregarded both by the pro-Knox field, and (partly) even from the prosecution. But it is something huge and would deserve a chapter on its own. There is crushing evidence that Knox told a load of lies from minute zero of the investigation. Evidence of lying does not mean automatically proof of being a murderer, but this is one solid piece of circumstantial evidence, a "system" or an evidence area on its own that stands like the elephant in the room.
7. The unexplained false accusation of an innocent (for this, Knox was definitively found guilty). I say "unexplained" meaning unjustified, I mean that this is a furthe area on which Knox opened an additional chapter of changes and lies. Actually, the manifest inconsistency of all the claims of "coercion" or "false memory", the logical analysis of Knox's inconsistencies about the topic and the crossing with others' testimonies leave asolutely no reasonable doubt on this point.
8. The bathmat print. This print is compatible with Sollecito and not compatible with Rudy Guede. It's a rather obvious piece of circumstantial evidence in my opinion.
9. Sollecito's lies. They have a lesser weight compared to Knox's lies, but several Sollecito's statements are a tool for crossing evidence, that helps to illuminate some of the severe inconsistencies in Knox's stories.
The above areas are not all. There are other sets and other classes of evidence, it's just a summary scheme as an example to how this case appears to me.
)) talking about as I'd wondered if I'd gotten lost during the time I was having trouble following the thread. First off I'll re-post the translation you produced (is this your translation BTW?) regarding Vecchiotti in court, in September of 2011, in reference to the negative controls:
