By all ebooks?
Good question. Actually, others I have done in a previous ADE version 2.x display the cover image ok, but this was the first time I used ADE 4.5 with the same InDesign CC 2014 export to EPUB process to make EPUB 2.0. iBooks shows the image, so I know it is there somewhere!
O.o Then its possibly better that you ask the again with InDesign Forum. Its seams a error in the settings of the export.