In addition to what others already said about the commercial version of TETRA (which is where the backdoors were discovered), the cops, by the nature of their job, do handle a lot of personal data, and transmit them over the radio. Theoretically, there should be safeguards in place to prevent misuse of the data handled by the police. In practise, this works more or less well, but at least there is some attempt at oversight. If their radio systems have a backdoor, every Tom, Dick, and Harry (and Sergeij from St. Petersburg) can get access to those data with exactly zero oversight and safeguards.

Additionally, among the noncommercial users of TETRA radios are lots of other emergency services, including ambulance services. That data concerning people's health are of a somewhat sensitive nature, and therefore need to be protected from unauthorised access, hopefully doesn't need any further explanation.