When trying to obtain evidence, investigators or regular officers will make frequent recourse to lies and intimidation to get you to admit to things that you may or may not have done. For example, "If you don't tell us where you were that day, CPS will take your kids away" or "Look, if you just admit what you did, we can let you go" or "We've already detained your wife/brother/mother/father and they've fessed up; just yadayada."
Police are trained to lie to you in the course of investigation so “they could choose not to do their job (by conducting an investigation)” doesn’t refute the notion that it’s their job to lie to you, it affirms it. It’s like saying “it’s not cops’ job to lie to you, some of them are dogs whose entire job is sniffing out cocaine with their extraordinary sense of smell”
It's like saying it's a pilot's job to wear a uniform. The uniform may be required by their employer, but it's not the job. A law banning pilots from wearing uniforms would not result in jail time for doing their job.