Using Concord on your site? Offer a link to the source. Modify it to work better with a framework? Offer that modification too. Build an entire product off of it? Well, be prepared to release the source to that product under GPL as well. If you don't like those terms, feel free to start from scratch and not freeload off the original authors who were kind enough to share source with you in the first place.
I don't understand how people can pay good money for things like game engines, then turn around and think that OSS doesn't have a price too, or get upset when they are asked to pay it. You just end up paying back to the community instead of into a company's coffers.