I cannot agree with You in 100%. You are talking mainly about crap(like SuperTuxKart which is unplayable, much more worser than original), or PPRacer(maps made by kid or /dev/urandom, not like old TuxRacer one), look i.e. on Joomla(fork of commercial Mambo). I know that is a bit different/complicated story(fork was made by coworkers, that wanted to develop in the clear way.. There was also many GPL soft(mainly linux/busybox/mplayer illegal forks), by many leading companies like Cisco, Kiss, Ericson....but they were illegal. Now with GPL is problem with server side software(you must distribute source only while you are distributing binaries), so theres AGPL.
I really like quake2 forks like Warsow or Nexuiz(theres some Quake3 like Tremoulous), but they are much to different(maybe better words is "based on code") form originals, so they cannot compete.
Im really happy that game is using Free/portable libraries, and give source so we can run it on any arch/OS (remember Stallman and new closed HP problem, theres no problem with that license). Bigger companies probably will not use GPLed source(because they will be not able to close it), but im use that there will be(if game will get more popular) many shiity "mods" like NeochildWars_blinkig&shiny_ALL, or Teewars_AutoAim_MegaWarezHack_by_5UCK3R, but providing any source probably will cause it(they feel 1337 h4x0rz, because they can show friends sth popular with their nicks). But not providing source will be like move excluding users of exotic Operating Systems, and procesor architectures, like latest Soldat with PB not working in WINE.
Im a Linux/FreeSoftware Extremist(using gnash instead flash , but I know that you written this game from scratch(including artwork), and game for me is definetly perfect(gfx/music[maybe sometimes annoying]/gameplay), and Open Source(resctrictive licesne, but avaible), and I really dont work to anybody get profits form Yours hard work, and trying to destroy Yours project to make moneys, or some idiots chalanging in mods and stupidity, so definitely BSD or ZLIB like license is BAD, because they ALLOWS TO CLOSE SOURCE of forks. BSD is Public Domain like, ZLIB force only to return diffs to modified code(only your code, not their around yours which will be secret). LGPL allow for dynamically linking with closed source, so best for You probably will be GPL for game and AGPL(for server, will not allow to close network protocol) mix. Delaying in time relicensing Artwork can be good idea, until game(and their name) will gain strong popularity(or project will fall), so anybody will recognize any fork as Your game. I also think that any fork w/o Yours artwork will be useless, because there many similar game, but any has so HQ artwork/animations(i love chain reactions, and moving). ID software also provided only source for Quake engines under GPL, Eternal Lands too. And deciding voice gaved Stallman saying that Art not must be free Its Your decision, I wanted only to share my Linux/OSS experience, and Im not against Public Domaining it. (now please give me armor to protect from FreeSoftware Zealots
//SuperTux is best Mario/JazzJack Rabbit(story based single player) replacement, now we has also network mode JJRabbit replacemat, which was playable trouch WINE, but 640x480 was frustrating. I saw some "Race" maps, why theres not downloading maps from server option ?