In the old version there have been lots of cheating issues. People manipulating network packets, using memory editors and so on. Some people are kinda used to this from other (flash) games they play. One guy cheating can ruin the game experience for 10-20 people. If I'd make the code open source (especially the server side that can't be obtained this way and renders decompiling useless), I'd tell all those cheating people how my protection algorithms work and "may" render them useless in the long term. Imagine world of warcraft being open source. I don't like this also, but it has always been a problem and I really don't want to let this happen anymore. The problem is not just the people having an unfair advantage but also the people doing stuff that may cause other player's clients to crash or something like that. An example: In the old version there has been a player creating games for 99 players, just because the code was bad. This resulted in a complete loss of points and skill for players 5,6,7...etc. This is not possible anymore, however I want to be better protected against that stuff. Instead I am going to provide an API and the possibility to extend the game by adding maps, image themes and sound sets on your own.
Sure, I don't ask money for the game, but there are people that are not happy with me developing it (won't tell the details). So I am doing everything possible to give nobody a reason to sue me with anything. If I'd make money with the game, the situation would be different, because in this case, I could spend the money I earned on defending the game against everyone who aims to harm it. But I don't, and I am still responsible for it. Currently, if someone would attack the game in any way, I'd use my company's personal and monetary resources to defend it to the maximum extend possible. However, I don't want to waste my powers on that, I just want to develop it farther and farther - for you, for me, for fun

The new license does something similar. If someone creates a map, the contributor states that he does not infringe anyones copyright (e.g. for the background image) and by doing so, I am not the responsible person if this happens anyway and I really didn't know. Furthermore the contributor grants me the permission to use his contribution inside the game so that he cannot decide to sue me on some random point. The license aggreement is quite the same one that
Netty, the underlying network framework I added, uses. So nothing special, nothing bad
