Interesting article Ne1L C. Lest I be answering my own question, here's my best guess about its rationale and please feel free to correct me if I didn't get something here:
The hard break in the US might stem from the unique relationship between local stations and national networks. Unlike everywhere else, US networks don't have centralised control over a station's operations, not even among the stations they own. On BBC One and ITV1 for instance, the regional programming is very limited whereas in the US, the networks and local stations are responsible for roughly half of the schedule. And I think the hard break might be a way for networks to cede some control to local stations to allow them to insert local content (ads, local/syndicated show promos, etc) for a minute or two.
My feeling is the soft break is a break at the discretion of the show's producers. The ads and programme promos seen there appear to be technically segments that are queued up in the show's galleries (not the master control).
This is probably why it may be best to watch US election night from cable news networks than broadcast networks.
Hope I got it right.