I think it's because the authors consider themselves artists more than businessmen. I mean, it would make sense to see people enjoy your art and think you're awesome.
No, I think it's because they are adapting to the reality of the situation. You aren't going to stop the pirating, at most you're going to slightly inconvenience them (and possibly majorly inconvenience your customers). Their efforts on the site at worst will have no effect (other than they generate a few license keys) and at best, turn some people pirating into customers; if not for this effort, then for their next. I don't do it, but I know people that do- and most of them are reacting to something; prices, bad games, bad DRM, companies shafting them (GOTY editions really pissed a lot of early adopters off- especially when released at the same time as expansions or DRM). Many of them, once the condition that causes it goes away, actually buy the game.
Is it ideal? No. Do I agree with it? No. But either you adapt, or you become the RIAA/MPAA.