In most cases, one reason they can is because there is no enforced repercussion for doing so. Most police forces do in fact have rules for this type of conduct

In most cases, one reason they can is because there is no enforced repercussion for doing so. Most police forces do in fact have rules for this type of conduct and institutional mechanisms to regulate it, but enforcement has become discretionary to the point of non existence. Please note that this description is extremely generalized and that there are numerous other factors.