In a department as large as Memphis has, there are bound to be numerous non-driving jobs for personnel...jailer, latent prints, etc.
Both Tennessee and Alabama use deferred prosecution in courts for DUI on a regular basis. If the officer is eligible, a deferment would allow the policy Memphis has adopted to work. Notice, the article states "arrested for DUI", not convicted. That indicates to me that a deferment may be what gets used.
In a smaller department where everyone has to drive for patrol or traffic enforcement purposes, this policy could not work.