exponential scale

Ive been doing the earth zoom tutorial and I decided to experiment and use the same technique with some solids.  In the tut he uses the exponential scale on the keyframes and so I went to do the same thing with my solids and it was greyed out. It became available once I made them 3d.  How come? 
In the tut he didnt make the earth images used 3d yet exponential scale was not greyed.