It makes a bit more sense when you realize that the Empire saw themselves as the continuation of the Republic (the Senate wasn't abolished until partway through A New Hope, for example), and Obi-Wan was very much a part of the Republic's leadership, being a sitting member of the Jedi Council and all.

So it's not so much a "villain accent" as it is a "Republic leadership accent", where the Empire is just a corrupted Republic.