There is already a precedent to give amnesty and/or immunity for one crime in certain circumstances but not have that extend to future crimes of a similar nature.
It's often used in criminal cases. The 'lesser' criminal will be given immunity in exchange for testifying in court or giving information in order to secure a conviction against the 'bigger' criminal. It doesn't mean that the lesser criminal can then go on and commit the same crime and not be punished.
Similar with amnesties, if you came forward during a weapon amnesty and handed in your illegal firearm, that does not mean that if you found with an illegal firearm in the future, you will not be charged.
So, if a subletter comes forward this time, they will not be charged. If they are caught subletting again, they will be.
If an illegal migrant comes forward to give the names of his friends who died in the fire, he will not be detained and deported this time. If he is then caught working in the local chippy and still has not put forward his refugee claim, he will be detained and deported.
Although, I would personally like to see all victims be granted asylum in the UK. They have suffered enough.
The subletters should face the consequences if they do it again.