Judge Mark Brown, sitting at Liverpool Crown Court, said the recent changes to the law that increased the maximum sentence for people who allowed a dog to carry out a fatal attack on a person do not apply to this case.
The tougher penalties, which mean dog owners can face 14 years in prison, were brought into force last month but Sulley and Woods' offences predate that ruling.
The judge can only sentence in this case according to the old guidelines and not retrospectively.
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Marjorie and Michael Cawdery, both 83, are understood to have been stabbed to death in a killing their family called "incomprehensible".
Detectives have launched a murder investigation following the shooting in Brent, north west London on Friday night.
Hot and humid throughout the Bank Holiday Weekend.