And even in the worst places, you can trust that casual criminals will make far less obvious trouble right next to the cops and make them look bad.There are multiple other reasons.

On #3 there is no such thing as a National Criminal Search unless you are law enforcement.

Would you take someone who work as case manager at criminal justice as tenant?

This can very frequently have legal implications stemming from : fines, suspension, criminal charges, etc.

NO simply not the best plan .. you have no control of who is occupying your house. could be 2 people, could be 20,, you can't screen for criminal, and even if company says ALL their workers are employed checked..

Criminals don’t care about rules/laws.

Even in localities with "income discrimination" laws, you can still obviously screen tenants for prior evictions, criminal history, charging a higher deposits, etc. 

Here's mine as an example:Tenant, his guests or invitees, shall not engage in criminal activity, including but not limited to violence, threats of violence, unlawful discharge of firearms, or drug-related criminal activity.

I do credit screening last, after the basics, including a free criminal back-ground and sex offender check.  

@Ras SchusterRas, you are correct that under the new Act 176 that a landlord can now terminate any tenancy when they have reason to believe that a tenant is engaging in criminal activity, drug related or not.