You downloaded this game from the official website for free. The developers didn't make it mandatory for you to pay to play the game. Guess what ? It's freeware.
I'm not flaming here. I'm sure he plays the game while knowing he hasn't paid anything. What kind of people use something, and then ask if it's free later ? Thus the question, does it matter to you if it's freeware or not ?
He was enquiring what aspects of tremulous needed to be added to the gpl-ed q3 engine to make trem free. This is not asking if it is free, this is asking what aspects of tremulous made it possible to be be downloaded free of charge and doing so because he wonders why not more mods go standalone.
To answer his question. The source itself was gpl-ed and thus free, no q3mod has to worry about that part.
But the assets (textures, models, maps, sound etc) that came with q3 were not part of the bargain. As such any mod that wishes to go standalone needs to make their own.
They cannot use the copyrighted material which may be used by mods. Why may mods use this? Because to play mods you still need to buy the game and thus you buy the right to play with those assets included.
It's a lot of work making those assets and it costs a lot of time.
Also sometimes the mods don't really allow themselves well for standalone. CPMA for example is a mod that is based around the q3 gameplay and maps and to make it standalone would have to use something like OpenArena. But that project itself still does not include many of the maps which are favorite among the fans.