Gee... You assume laziness when that may not be the case at all. What if they just have a hard time remembering? I have a great "radio memory" as far as music goes. If I've heard the song a few times, then I can usually play it pretty accurately. I've been on stage and play songs that I've never even practiced at all simply because I can hear the song in detail in my head. I sometimes get annoyed that others that I work with don't have the same ability. Right now, we're adding Toto's "Hold The Line" to the set list. The keyboard player, a fine musician in her own right, has been having a time of it getting her parts right. I get a little annoyed, but very much keep it to myself, because I understand that I've heard the song many more times than she has, and I have an extra ability in this regard that she doesn't seem to.
Some people have memories like that for lyrics too. I've worked with guys and girls who could look at a lyric twice, and then remember it almost perfectly from then on. As far as lyrics go, I, regrettably, am not one of those persons. I very much wish i had the same gift with lyrics that I do with music... But I don't. There are time where I should use cheat sheets on stage, but I don't because I have a very difficult time reading them while playing.
For some, yes, it may be laziness... For others, it may be a crutch to get over the nervousness of being on stage, or simply that they don't have very good lyric recall. Don't automatically assume it's laziness.
PS. We will be performing "Hold The Line" for the first time live at a party this week end. I will almost certainly have my cheat sheet for that in front of me.