I've had this thought as well. My guess is that former meat eaters want to experience some of the things they did before they stopped eating meat. I mean, a hamburger is a pleasing thing to eat. It tastes good. There's no shame in admitting that. I can understand wanting to enjoy something that you like without the aspects of it that you don't. It's the same concept behind diet soda or sugar free candy, I suppose.
On the other side of the coin, the fact that the very existence of meat alternatives seems to piss some people off to a large degree, is bizarre to me as well. There's clearly a market for it. If you don't like it, don't buy it.