It is inefficient, but it is futureproofing in case scope does change some way down the line. Easier to slap six seats in than to buy new hulls. American have been removing seats from CRJ700s to get them to comply will "small RJ" on scope so they see some merit to operating them in slightly less efficient configurations if it allows them to stay on property.

The only way I see scope changing to allow more large RJs would be if small RJs were parked in their place (not 1-1 ratio), and also for JV scope improvements.

The ex-Comair 700s were 2002-04 vintage, and probably 20 year hulls, so they can stick around until early 2020s.