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That's the weird little secret of true two pipe steam systems! Putting a vent on what might just as well be an open pipe started when people figured out clever patented widgets to occasionally pressurise the dry return to keep the pressure differential between the returns and the mains to some reasonable value — to keep the water in the boiler where it belongs. (The Hoffman Differential Loop, for instance, was advertised as "The Watchman of the Waterline").

Then people realised that if you put a vent there and steam got into the return from, say, a bad trap then it would close and you didn't get steam in the boiler room and the building super didn't get complaints about steam in the basement. Great! Doesn't help the heating much, but… who cares?

Funny world.

but any water that gets in to it may not find its way back in to the main but may drip out