wcastle
For the same reason other Roku-developed/managed/operated apps/services/functionality "keep coming back" if removed (e.g. Howdy, Photo Streams, Live TV Guide, The Roku Channel, etc): Roku spent/spends significant time/effort/cost developing/maintaining/operating the services/apps/content and therefore wants you/us to use it.
You also agreed to this behavior when you when you consented/agreed to the Roku account/device Terms: We (or our content providers) may update, add, or remove channels, content, and/or content providers from time to time (possibly without notice to you).
There is no point complaining: Roku isnt going to stop developing apps/services/content (and auto-(re)installing them from time to time if removed) unless and until they go out of business.
Just leave the Backdrops app installed and move it to the end/bottom of your apps list to prevent/minimize unwanted app list changes (I suggest moving all the unwanted/unused Roku apps to the end/bottom of your apps/list).