Let's get this in perspective: Police Officers would never be kept in the Force had they committed serious criminal offences - rightly so.
What's being referred to in the article and question is the matter of "spent convictions" which would relate to minor misdemeanours. Nobody is talking about murder, rape, paedophilia, armed robbery etc etc.
Police Officers are as human as anyone else in society and just as prone to temptation and error. These human frailties may not necessarily prevent them from carrying out their duties fairly, impartially and honestly even if they themselves have tripped up along the way.
Besides, when all is said and done, the Police are merely servants of society which is itself made up of many disparate individuals, some of whom not only hold down far loftier jobs than that of a Police Officer, but who have been convicted of some despicable offences without prejudice to their livelihood.