Hmm. The geometry of ATCS is simple enough for you to make a .map from scratch by looking at screenshots (as previous mappers have done), with potential room for improvement considering the map-related bugs I've experienced while playing in ATCS. A good mapper will do a far better job of reverse-engineering than a decompiler will.
As for the whole "open-source spirit", the game source code must be available, but not due to a fluffy emotional ideal. It's because the project must adherence to the license terms of IOQ3. There is, however, no such obligation to release the source of the game assets - in this case, that is the original mapper's decision.
