The issue was recently brought to light by the Nashville Business Journal, after David Arnott discovered hundreds of photos he thought he deleted preserved in his Google Photos account--all taken weeks after he uninstalled the backup app. It's not a fluke, either: I tried it myself, deleting not only Google Photos, but also disabling my phone's Google Drive and Google Plus apps, and my phone still backed up every photo I took. All six of these photos uploaded, even after I deleted the whole suite of apps:
It's a quirk in how Google Photos deals with upload settings--instead of keeping it all localized within its own app, Photos farms out the task of managing uploads to the Google Settings app, which also handles location data, search functions ad settings and more. When you delete the Google Photos app, its settings live on, as do whatever backup settings you chose when you first installed it. It's easy enough to disable (just pop into Google Settings and tap on "Google Photos Backup"), but it's not very intuitive.