You can continue to use your rank as a title after retiring. You have to make it clear you are retired, however, usually by putting (Ret.) after your rank. He did this in his twitter bio, though not his username. I guess this is ok as I could call myself @CplCuntCrusher and not break any laws or regulations, even having never been a corporal. Davis, being an actual retired officer, using it in his handle seems disingenuous (and also shouts "I used to be someone important! Listen to me!")