Arbitrarily? Source, please.
Did the orignal poster say "arbitrarily" anywhere in his reply? Or did you just arbitrarily altered someone else's reply?
To use your own quote again "As we have seen the basis of Common Law was custom. The itinerant justices set out by William the Conqueror examined the different local practises of dealing with disputes and crime, filtered our the less practical and reasonable ones , and ended up with a set of laws which were to be applied uniformly throughout the country."
filtered our the less practical and reasonable ones certainly sounds like (and deciding which local custom, practices, etc., to use and not to use).
"Common law is a result of a natural order which first solidified into custom and then into law. Primitive man knew nothing of laws, all he knew were customs which eventually evolved into rules of living. While no one can point to the origins of our traditional moral rules, their function in human society should be evident. These moral rules, or traditions, are necessary to preserve the existing state of affairs; such that culture was allowed to evolve, and in turn, with culture, civilizations arose (Landry, 1997, p. 4). The Royal Courts of the thirteenth century were confronted with cases for decisions. There was no uniform body of statutes to rely upon. The only available source was local custom. It did not take many generations of Royal Judges, dealing with actual disputes, to establish the local and divergent customary laws into a single jurisprudence which was the Common Law of England (Read, 1955, p. 42)."
http://www.iejs.com/Law/origins_of_common_law.htm