[Mr. Mac]

Version #8 of the Rules clarifies this point: incidental pushing or plowing of CRATES is inevitable and expected--the refs will not call any penalty.
If a robot is (in the ref's opinion) deliberately attempting to control the movement of a CRATE by pushing it along the floor, a penalty will be called (regardless of whether the robot is carrying a CRATE or not.)

Obviously, a robot that has a plow built onto its front end will have a harder time convincing refs that their pushing of CRATES is unintentional!