A) The police don't get to make the rules of the country, that is up to the public, politicians and top level legal experts to devise. The police enforce the rules of the land, no matter who they were brought in by, with the end goal being for the good of the general populous.
B) The fact you are arguing against criminalising legal highs shows you know nothing about the social problems they cause, the chemistry behind them or the laws that allowed these "tax paying businesses", as you put it, to operate with impunity. Their continued operation was a technicality of law, which was written decades ago when these sorts of modern adaptations to drug compounds weren't even envisaged, let alone for legislation to be drawn with them in mind. Laws need modernising every so often to allow them to keep up with how society is - right from the menial ones, to the very important ones.
If you believe in decriminalising drugs, then use your rights as a free citizen of this country to campaign for decriminalisation. You may even be surprised by the number of "pigs" who agree or partially agree with you...