I'm in no way defending the police in this, but the reality is that it's probably not high on their priority list. This type of directive needs to come from the top; perhaps if enough people write in about it it might get recognized by the administration and acted upon.

Police - JW (Communications) assigned this issue to Traffic - KC (Permits)

Thank you for reporting your concern to City of Cambridge via Commonwealth Connect. We will use this information as part of an ongoing review of data to better understand where and when to deploy our officers for this type of enforcement. In the future, if you see this type of activity, we encourage you to call the Cambridge Police non-emergency number at 617-349-3300 and an available, nearby officer will be dispatched to the location.