This is likely due to the fact that Benson could serve a three-game suspension for "player-conduct violations" during the lockout. Benson is planning on appealing his suspension but because his status for the next thee weeks is currently unknown, it makes sense that the Bengals are planning on possibly bringing somebody in to carry the load in Benson's absence.
In case you forgot, Johnson played for the Bengals in 2009. He played in seven games, carrying the ball 46 times for 202 yards. He was used when Benson was out due to injury and to give Benson breaks from time to time.
Johnson has racked up over 6,000 yards and 55 touchdowns in his 10-year career.
What's also interesting about this news is that it shows the team's lack of confidence in Bernard Scott even more, but that's another article for another time.