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.
More top news
A jury has failed to reach a verdict in Emile Cilliers' trial on charges of attempting to murder his wife by tampering with her parachute.
Prince Harry and Meghan Markle are rumoured to be heading for engagement. Is the institution on the cusp of change? ITV Tonight reports...
Average earnings by 2021 also look set to be nearly £1,400 lower than forecast in March 2016, the Institute for Fiscal Studies has warned.