I suspect that they do this so that people are not scared to phone them and report things.

It makes sense as if everyone thought that they could track them, no one would call. Even then I wouldn't be surprised if people track down the last known phone box in suburbia to make these calls... but they don't need to. Even if it creates a problem for the police, without this policy they would probably hear nothing.

www.crimestopperswa.com.au/about-us/anonymity-guarantee/