I believe that's a standard practice. The phone company tore up the sidewalk in front of our house some time ago to upgrade the lines, temporarily patched the trenches with asphalt, then removed the asphalt within a month and poured new concrete. I believe the asphalt is just easier to throw down than concrete (and a lot less dangerous than a sheet of plywood). If it stays like that more than a month, I'd try giving them a call.