The emphasis in 4.MD.1 is on “times as much” or “times as many”. Since a foot is 12 times as long as an inch, there are 12 times as many inches in a measurement as there are feet. The corresponding statement the other way around involves multiplication by the fraction 1/12, and so waits until Grade 5 where such multiplications are introduced. Thus you get conversion both ways in 5.MD.1. This is not to say students couldn’t use the table to convert both ways, but it is not required.