Yes, it wasn't like this in my house until some years ago when we had a small expansion constructed and the contractor installed these in the new portions just as a standard thing he does, and said he was retro fitting the rest of the house that way too. ( We know him, friends, so we know he wasn't just trying to get extra $ out of us)
For a few minutes I thought it seemed a needless detail, until I realized that often enough people forget to change their stand-alone unit batteries and maybe deal with the the occasional chirp in an "I'll get to it" mindset until it runs down completely. Having it tied to the mains solves that because it's relentlessly annoying to hear that chirp at random intervals, and the battery is a nice backstop failover from the mains going unexpectedly molten.
I mean, it is code now to have them hardwired up. Actually, I think the requirement might be for interconnectivity rather than mains power. It's probably both in some areas. It's quite possible, depending on the extent of your expansion, that he was required to retrofit all of the smoke detectors in the home.
For a few minutes I thought it seemed a needless detail, until I realized that often enough people forget to change their stand-alone unit batteries and maybe deal with the the occasional chirp in an "I'll get to it" mindset until it runs down completely. Having it tied to the mains solves that because it's relentlessly annoying to hear that chirp at random intervals, and the battery is a nice backstop failover from the mains going unexpectedly molten.