The reason is that whenever I start DC (and I have it active 24/7 on a disconnected Terminal session) and try to use the net I suddenly remember the good old 33.6 modem times
The feature could be as low-level in terms of DC source as possible; no distinctions, no exceptions, just a possibility to limit bandwidth usage to a set kbps, or perhaps a percentage of the maximum throughput possible.
Also, the feature would not be thought of as unfair, since it would slow down both uploads and downloads, by the same factor.