What is with the "pay per" for online RPG's?

It's because big games by big companies such as WoW (by Blizzard) have constant updates, and so I figure the developers think a flat fee isn't enough. They want you to continue to pay a subscription fee to cover the cost of their time that they spend developing new content and releasing periodic updates to the game. Another thing that my friend has experienced personally is that when you want to quit the game, you think to yourself "Wow, I've spent so much money these past two years on this game just to quit", and so it's sort of like an addiction thing right? You don't want to stop because you've invested so much time (and money!) in your online character that stopping is hard. That's why I usually shy away from pay-to-play MMORPGs.