Mike Bloomberg joined the Democratic race for President on Sunday, creating a conundrum for his journalists at Bloomberg LP. Do they quit covering 2020? Do they stand on the principle of editorial independence, while promising to treat their billionaire proprietor like any other Joe, Pete or Elizabeth?

Neither, per a memo Sunday from John Micklethwait, Bloomberg’s editor in chief. “We will write about virtually all aspects of this presidential contest,” he said, adding that a Bloomberg reporter has already been assigned to...