It's still available from Adobe but, as others have said, it's not freeware. Adobe have discontinued support for CS2, and the download is meant for people who have a licensed copy and may want to reinstall it at some point.
OTOH, Adobe put it on their website with the serial numbers and haven't tried to restrict/control downloads. CS2 is an old product and I don't think they really care. It's not going to impact sales of CS6.