Degrees of Innocence?

Tweet I really don't understand how the UK government can honestly justify waiting up to 12 years to delete the DNA records of innocent people following the ruling in the European Court. Far better the Scottish system of automatic deletion after three years unless a court order is obtained.

And although I understand that crime comes in varying degrees of severity, I don't understand how innocence is measured in degrees. Yet that seems to be exactly what the UK Government is proposing - DNA will stay on the database for a time period which depends on the nature of the crime of which someone is innocent. If someone is swabbed as part of an investigation into a crime of which they are completely innocent, how on earth do they have any control over the severity of the crime?