I believe that is normal. The A/C should be on also to dehumidify the air before it reaches the heater section. There the dehumidified air is heated, further dropping its relative humidity, and the hot, dry air is directed at the winshield and side windows to clear them up. Once the windows are clear, the defrost feature can be turned off and a "more appropriate for the weather conditions at hand" setting on the climate control can be selected.
I agree the selections on the system are not what I would prefer but they seem to do the job. Newer models have a sysem that is more readily customized for the user's needs, but they still limit the number of choices in ways that make me wonder.
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)