I've never seen them being accessible with a jack hammer every house I know they have a removeable cover. You're in a world of hurt probably if they do have to use a jack hammer and cemented it in (vs. covering it completely in sand and then pouring the concrete).
They're left exposed with a cover for a reason. Typically because tree roots have grown into the pipe and plugged it a plumber would run a device to cut the roots out down that opening. In my case my sewer wasn't working right and the plumber put a camera down the pipe at that location and we found a dip (probably a sink hole).
I've never seen them cemented in, I would opt it was a mistake. I'm thinking back and remember my father and neighbor had a plumber cut roots out using that opening, and me with my sewer pipe that dipped.