Usually a pardon is granted by the president AFTER a thorough background investigation is implemented by a special office WITHIN the DOJ.......even "recovering" racist Sessions knew this.....
But, the orange charlatan wanted to please the horde of slightly white supremacist base with something they could cheer the low-self-esteemed Trump.
Had a more thorough investigation been done, EVEN Trump would have had a hard time explaining this.
In 2007, during an interview, Arpaio openly stated that it was an "honor" for his department to be compared to the Ku Klux Klan, a white supremacist terrorist organization.