It's more of a business practice, basic principle of supply and demand. Just like any other application that also offers a a free version is up to the consumer to decide if it's worth using. Therefor it's not some of money garb technique when the consumer is aware of the benefits and agrees to it. Besides paying $5 is not a big deal. If you think about it we waste it or dumber things that we probably don't even need. I mean seriously if people are willing to pay for over priced food when they know they can't afford this is not as dumb. In fact your only paying a subscription for extra content that is in high demand. SE is not saying "if you don't have this app you can't complete the game", hence OPTIONAL.