Funny, my chickens are the same way. As far as I could tell, they just scattered the oyster shell around without eating it, but they loved some dried and crushed egg shell I threw in.
It appears to be the same here. The hens love (and devour!) the egg shells, and just play around with the oyster shelI. They may eating some of the oyster shell, because the shells on the eggs are more consistent and thicker. No more splotchy-looking eggs. I have always fed back their egg shells. I don't dry the shells out in any special way. Just let sit overnight and then flatten them out a little. I throw them in the run.