Here's my two cents on why double sleeving is a crime in the Altered Carbon universe. What is clear from the plot is that double sleeving is not just a crime - it's a deep cultural taboo.
The best parallel that I can draw is to the move "The 6th Day" in which cloning technology had advanced to the point that any living creature can be cloned and the clone given the memories of its donor; however, cloning of humans is forbidden by law, and violations of the law are called "6th Day" violations, which is a reference to the Genesis Creation Myth in which God created man and woman in his image on the 6th day of creation. Cloning humans is regarded by religious zealots as an abomination.
In the Altered Carbon universe, society has evidently gotten past any taboo against cloning humans and genetically engineering sleeves, but it hasn't gotten past the idea of the soul. Altered Carbon is perhaps intentionally ambiguous about what is going on when DHF is transferred from one stack to another and what is going on when DHF is backed up. Most of what we see in the story when DHF is transferred from one stack to another is that the DHF is REMOVED from one stack and transferred into a second stack. An illustrative metaphor is pouring water from a full bucket into an empty bucket - the water was in bucket #1 but is no longer in bucket #1 because it's been transferred to bucket #2. Thus, DHF is for all intents and purposes the human soul.
However, the story also makes clear that it is possible to copy DHF for backup purposes since it is essentially computer data. But, as we know, when one transfers a file from one computer system to another, the data on the first system is not necessarily deleted during or after the transfer. If DHF is really just computer data, then DHF can be downloaded and distributed into as many other stacks as desired. However, when it comes to double sleeving, copying DHF to the stack of any other sleeve and animating that sleeve violates the taboo because a soulless human being is now walking around the settled worlds, and that calls into question whether there is such a thing as the human soul.
In my opinion, meths who are real-deathed and are reanimated from backups are actually double-sleeving although it's not called than nor is unlawful or in violation of the cultural taboo. Nevertheless, religious zealots like the one in "The 6th Day" would regard a meth who is reanimated from backed up DHF as an abomination. For some reason that is not explained, the "legal fiction" or "moral fiction" is that a meth who is reanimated from backed up DHF is still the same person that was real-deathed and that the soul of the real-deathed meth now resides in the reanimated sleeve. However, backed up DHF that is transferred to two or more sleeves that are then animated is regarded as an abomination of the highest order and as a capital offense.
So, double sleeving directly undermines the closely held idea of the existence of the human soul and of continuity of life and consciousness of an individual from one stack & sleeve to the next stack & sleeve, Accordingly, a double-sleeved individual is an abomination that must be exterminated, and the perpetrator of the double sleeving is must be punished harshly. Other responders in this thread characterize this is terms of an undermining the idea of responsibility of the individual for his/her actions.
One question I had is why is it not possible to merge two or more DHFs from double sleeved individuals to yield a single DHF that has memories of both double sleeved individuals. I presume that the response would be either that doing this is not possible or that while it is possible, doing so would cause psychosis in the merged DHF as the mind tries to figure out which of the DHF is actually is upon recalling multiple memories for the same period of time. In "Total Recall," this sort of thing was called a "schizoid embolism" and why one was not permitted to go on a memory trip of something that he/she had already done in real life.