You are right. The reason behind it is that these companies use mitochondrial DNA for their analysis, which is only passed down from mother to child. /s
You're right that Ancestry.com is focused on mtDNA, but 23andMe actually uses a custom Illumina microarray with autosome, allosome, and mitochondrial targets. Here's a study where 23andMe used their (old version) chips to replicate a bunch of known genetic associations, most of which are autosomal: http://journals.plos.org/plosgenetics/article?id=10.1371/jou...
But why do they need to analyze the victim's DNA - wouldn't they typically be looking for a match for the perpetrator of the crime?
I guess if the victim's DNA were on the offender... But then why not ask the victim for a sample? :/
Because the victim is dead and they can't find any family.
Because the victim is a John or Jane Doe (i.e., they don't know who it is).
Except they analyse Y-chromosomal DNA as well.
But they aren't interested in the victim's DNA so that is irrelevant. They are interested in other DNA found at the crime scene, i.e. belonging to the attacker or someone related to the attacker.