Mold doesn't grow well outside of around 60-80F (15-26C), and it needs food. Steam and hot water systems also use treated water to help reduce corrosion in pipes. That said, radiators themselves still do corrode and create nasty looking water even if it's generally sterile.
You should be more worried about Legionella, but proper steam and hot water systems will stay above 140F (60C) as that temperature not only prevents the bacteria from multiplying but will kill 90% of them within 2 minutes.
Part of it is definitely dumped directly into the air through an over-pressure valve on each radiator. At least that's been the case in all steam-heated NYC apartments I've ever visited or lived in.
But what counteracts this is a loss in humidity caused by the heating of cold air from the radiator itself (cold air warmed up reduces relative humidity), and in my experience, this more than counteracts the little bit of steam hissing out of the radiator valve.
As a side note, I've always wondered if the steam being vented that way is actually coming from the steam plant, or whether there's a heat exchanger somewhere in between that isolates a building's steam network from the distribution one (which is the case for liquid water heating, as far as I understand).
nit: The valve is a thermostat that vents the air/steam in the radiator when it's not hot enough, letting new steam come in and replace it. This gets heat to the radiator quicker (over pure diffusion), and provides a mechanism of controlling its individual heat output. Of course, these valves are often broken.
Super heated water is a pretty effective disinfectant within the pipes. For interior spaces ventilation is a legitimate problem in non-forced-air systems.
