Not sure why... But when the circulating pump is working it will be pushing hot water through all of the little radiators feed by the individual zone controls if one of the zones is not asking for heat the little fans will not operate
In terms of logic and just speculating is that if the temperature is cold enough to warrant running the hydrohot for the house then two factors could be considered... One, the basement if it has a zone is intended to be heated and if heat is required and two heating the basement will assist keeping the upstairs heated.
I have only seen the front zone coupled to the basement. Generally the front zone is more of a challenge to heat...
I will tell you the first time I came across this it took a while to figure out what the deal was.
Johnny