Logically, BEN Kenobi is not a name to hide him from the empire.. but instead is to hide his JEDI name Obi wan, from the jedi hating dwellers of tattooine. remember in ROTS, the emperor makes out that the jedi are evil. (Same reason Droids are disliked in the OT. Tarred with the evil battledroid brush.
The prequels sure had their mistakes, but people seem blind to the genius behind the great parts.