The idea
s7 Drugs Act 2005 amended s63B Police and Criminal Act 1984 which had previously allowed police to require that any person charged with a 'trigger offence' provide a drug test. The amendment allows that now a test can be required of anyone on arrest, whether he is late charged or not.
By all means revert to test on charge – at least that has some validity
Why is it important?
This could lead to the bizarre situation whereby someone is arrested on a spurious allegation which, after a brief enquiry is found to be entirely groundless and yet, if they have refused a drug test on arrest, they could be charged with an offence with which they could, at least in theory, be imprisoned.