I don't know the actual reason for the switch over but this is entirely wrong and shouldn't have happened.
A census is - as you say - an "event". It has both a place and a date associated with it. It should, therefore, be recorded as an event.
To use the census address as a residential address is an error because many people were not present at their normal residental address on census night. They may have been visiting, lodging, working, travelling, etc.
I don't use Ancestry's trees. Is this reproducible? I'm wondering why no one else has complained about it. Can you add precise details of how you added the census events to your tree?