FWIW the battery api was first specified when people were still hoping that building phone widgets, launchers etc would be done with web technologies. For such usage, having battery api access in a standard way was of course a necessity.

In a wider sense, there are legitimate uses for many of these apis. To me it seems like the option would be that some phones would get these apis patched into the browser by the manufacturers instead, and that would run an even higher risk of security problems than when the browser vendors do it.

If you don't like your browser application having access to certain hardware in your phone, you can just revoke the permission, and it won't have the access (at least on Android, I have no idea how iOS works in this regard).