Destecado - you are ignoring the fact that outside of Imperial Law each world has its own sets of laws. Granted on a hive world a human life is cheap... but on a civilized world a human life is a little more valuable and they would probably have rules pretaining to human life.
You are correct that the value of life will vary from world to world and from society to society. For example the worlds of Ultramar place a higher value on the rights of the individual and on life than say on Necromunda. I was not arguing this point.
I also know that mercenaries do not operate under the Genieva Convention. I was merely trying to point out that "sanctioned" mercenary groups must operate within a given set of rules if they wish to continue existing. A mercenary force walks a fine line between legal and what could be considered illegal actions.
The rules that they follow are there to protect them as much as to protect the average citizen. If you are uncomfortable thinking of them as rules of engagement, think of them as stipulations of a contract between employer and employee.
Let us say that you hire a mercenary group and then order them to commit genocide on the local populace. without rule or guidelines implicit to the kind of work they can be ordered to do, they can either follow the orders or be in breach of contract.
Let us also say that an employer hires a mercenary force and then uses them as cannon fodder to support their own security forces. Without stipulations in the contract, this again could lead to a breach in contract or having a good number of the mercenary forces descimated in the action.
The rules and guidelins are there to make sure that the employer bargains fairly with the mercenary group and also holds the mercenary group accountable in certain situations while freeing their employer from obligation. Being part of a "sanctioned" mercenary or ganization is as much about knowning business as knowing the arts of war.
Also Mercs do not really have governing rules - they are not covered by the Geneva Convention. They do not have governing rules (even in our world) they define their own ROEs. They do what it takes to complete the task given them by their Employer. They do not have to be adaptable to all kinds of situations - they do what they need to to get their money. Their is no merc code - they set their own codes.
If we were discussing an "unsanctioned" mercenary group, I would agree with you. They would not be held to the same standards as a sanctioned group. They are hired to handle the less savery missions.
Unfortunately this cavalier atitude would not fly for a "sanctioned" mercenary group. Now you may ask, why would a mercenary group even consider becoming sanctioned or chartered by they Imperium. There are several reasons really.
1. Greater Bargaining Power
As stated in your post, mercenaries have no governing rules and are therefore subject to what ever payscale they can bargain out of their employer. They of course run the risk of having their employer not pay and instead try rubbing them out.
A sanctioned mercenary organization still trades on its name to secure jobs, but also has the backing of the imperium to back up their reliability. It is similar to the differance to using a bike messanger or a bonded courior to deliver packages and information. The mperial sanction marks a certain standard of dependability or atleast of professionalism.
The type of jobs that a sanctioned mercenary group can expect to acquire will also be different from those of unsanctioned mercenary groups. They will tend to be on a larger scale and for longer duration.
2. Procurement of Equipment
Being sanctioned, a company may have access through their employer or the Imperial supply chains to weapons, vehicles and other equipment, that they may be able to buy in bulk. Unsanctioned mercenaries would be relegated to buying from the black market or outright theft of material.
3. Legal Implications
I don't disagree that laws vary from world to world or even between governments on a single world. The basic contract with an employer is just the "boiler plate" that governs all contracts of this nature. Any other specific stipulations or guidlines that pertain to a given job may be negotiated between the employer, the mercenary organization and any govering body that regulates such mercenary contracts.
The idea is to provide legal protection for the mercenaries while working for the company or government body. Unsanctioned mercenary groups have no such protection.
4. Career Advancement
Sanctioned Mercenary organizations are in it for the long haul. If successful, they might be allowed to grow to the size of a division or more. Once leaving field operations, and individual merc could move into the upper echelons of command or maybe into a logistical or support role. Unsanctioned merenaries really have no future beyond their next job and acquiring enough money to perhaps retire on.
One last thought. It has been said that all mercenaries follow no guidelines or have their own rule of engagement and to not proscribe to anyone elses rules. While there are mercenaries that in deed operate in this fashion, there are also profession soldires and organizations that fight not for a country, but for money.
A couple of examples of this would be the German Hessians of the revolutionary war period, mercenary companies in employ of the Italien city-states of the Renaissance and in africa state sanctioned forces from South Africa, Ruwanda that are hired to fight in other African States that do not have their own standing army.