I guess the most likely answer is that whoever got assigned to do that cover wound up snatching an image from some "Top Ten Worst/Best Gay Moments In Comics" list without looking very closely at who the characters actually were. Though, really? Alan Scott and Sam are pretty damn obscure at this point; DC seems to have done their best to kick sand over that whole sorry blip on the radar. I wouldn't be surprised if that picture was included deliberately just to see if anyone in the Marvel offices noticed.
ETA: Yeah, the artist yoinked an internet image and didn't check the character names closely.