I'm sure that's true sometimes, but I dunno, I guess a responsible reader who understands the importance of feedback will make the time to come back and review if they really don't have time when they finish reading. As we all know, it takes SO much more time to actually write a chapter than to read one, a quick review is the least a reader can do to acknowledge the time and effort the author put into writing that chapter for their entertainment.
From the writer standpoint, we write for free because we enjoy it, and we offer our stories for free because we want them to be read and enjoyed too. From the reader standpoint, we're getting free reading material from writers who do this for the fun of it; the least we can do to show our appreciation is review the work we enjoy. And AC makes it so quick and easy to do that, there's really no reason not to.
I'll get down from my soapbox now and say that I don't review every piece of writing I come across. A lot of the times, I don't because when it comes to reviewing, I try to follow the Thumper rule - if you can't say something nice, don't say nothin' at all LOL. But any story I read more than a couple chapters of, I do review, even if that means coming back to it later. I just think it's rude not to.