DC had a different royalties scheme for stuff published in the mid-70's to '92. It's why they ran unrelated reprints to finish off the first JONAH HEX Showcase. That's known fact.
The theory you speak of is that they're making these solicits to gauge the demand for these particular Showcases; ergo, whether the demand meets the higher royalties associated with collecting these runs. CAPT. CARROT and SUICIDE SQUAD? Probably can. The others? Not so much.