Criminals, incarcerated or released, do lose some liberties. However they do no automatically lose all 1st Amendment rights. The Supreme Court case Turner v. Safley created the "Turner Standard" that asks "whether a prison regulation that impinges on inmates' constitutional rights is 'reasonably related' to legitimate penological interests."
For instance, prisoners are generally allowed to practice their choice religion, send as well as receive communications from the outside world, or seek redress of grievances from the government. Prisons may limit some aspects of these as they fall under penological interests such as maintaining order and security.
In Chief Keef case, yeah, he looks to be generally to be a piece of filth. However he's currently not convicted of a crime that he has not served his sentence for. Being a piece of filth also doesn't revoke your constitutional rights.