The problem the watchtower has, and the error being made here, is that they don't see the difference between the mosaic law and the Ten Commandments. They are two different things.
The law from God, the ten commamdments, was perfect. It was something no man could adhere to as one of the commamdments was to not covet - something everyone does at one time or another. It was a law from God.
The Mosaic law on the other hand was MOSES covenant as the Christ (anointed one) with the Israelites. This was the covenant which Jesus undid. Being the messiah he was a Christ (anointed one) with much more authority than Moses. So he replaced Moses covenant with his own, but did not undue the Ten Commandments. He did however fulfill the Ten Commandments, which only he could do - perfect man fulfilled perfect law and replaced the faulty covenant (mosaic law) referenced in Heb 8:7.
EDIT: actually I should amend something. I guess there is technically no error here - scripturally yes, the mosaic law/covenant was imperfect and faulty.