That's just the way it is on the HMC150. On later cameras they opened it up (HMC40 and AF100 have DRS capability in all modes) but on the HMC150 and HPX170 (which were the first low-cost cameras with DRS) it only works in 60p or 60i (or 50p/50i). I believe Jan said something about memory being a factor, but I don't know the exact reason.
Many thanks for your info here Barry, it would seem from reading other posts here that what you don't know about these cameras probably isn't worth knowing..