They might be offering it to bring traffic into their forum or websites, hoping that using a free domain name as bait will capture some people and keep them in their forum/site.
They might just be very generous.
They might be looking for something in return (say, with help on their forum, making a logo or something similar) - so not exactly 'free domain' then is it?
I'm not sure what people do these days but a year or two back they would register the domain in their own name and let you have the domain but they still owned it and it was down to their discretion if they took it off you without warning or some such. More often than not though, these type of 'deals' were fine.
Also, if something seems too good to be true, then it's likely it is!