Actually it makes sense if the aircraft are paid for.
On VERY short stage lengths (and the shuttle is VERY short) fuel burn is less of an issue. Furthermore with the shuttle (hourly service) they promise to roll out another airplane if one extra passenger shows up on the hour (rather than make you wait for the next flight) So you have to have a lot of aircraft sitting around as spares, and the only way you can afford that is if there is no mortgage on them. In that case fuel doesn't matter.
Infact the shuttle was originally set up by eastern to make use of redundant electra's in the same manner.
So you have to understand exactly what the delta shuttle is before you make those comments. We aren't talking about Song or delta express or anything other southwest clone. We are talking about the NY washington and boston shuttle pioneered by Eastern Airlines.
CHeers
Wino