a swiftly-passed law
Back in 1997 a solicitor explained to me that when DNA profiling was first introduced, parliament had been told and expected that it would be used exclusively to tie suspects to specific crimes; or to eliminate them from the enquiry. The use of the DNA database for 'trawling' was specifically not allowed; and it was expected that samples and records would be destroyed in due course some time after collection.
After a few months a slight modification to the statute passed through parliament, which most MPs probably didn't understand or didn't even notice. This allowed retention of the data "for statistical purposes".
And the authorities hiding behind such weasel wording to allow function creep wonder why there isn't too much public co-operation and why witnesses to crimes often don't willingly come forward.