I dont quite get the concern about delaying SS2 testing. The SS2 tests are still very infrequent and I doubt that the WK2 will be used fully by them. I might be underestimating the amount of service hours it needs, though.
I would understand concerns about the DC being able to damage the WK2. That would however be in stark contrast about the versatility VG claimed for the WK2 in the beginning of the development (it was advertised as being useful for just this thing) of the WK2 SS2 system.
Still, with only one WK2 being currently operational, even a freak accident damaging it would indeed delay SS2 testing. So maybe that is their reasoning behind that?
Could they possibly see DC as an airlaunched hybrid-powered competitor to SS2?
IIRC, the suborbital version of the DC was not meant to be airlaunched, but would have launched vertically like the X33.
Last time I saw that was a long time ago though and it might have changed since then.
Anyway, this does sound like a plausible explanation to me. But then, I dont think that SNC would care much about flying the DC suborbitally, if they were selected for commercial crew. I also think that it would need some modifications for that (IIRC, I would have needed more of the same hybrid engines, but that might have changed as well). So not letting them do the drop tests might actually be counterproductive, since it would reduce SNCs chances for commercial crew and thus would make a suborbital DC more likely. Granted, this is massive speculation on my side and I may be wrong here (it is still fun to speculate though).