I can only really define the behavior, the why is going to be because the dev felt like it seemed rational when he did it. It’s just a game parameter you get used to after a while.
Road riles are considered nature tiles, similar to grass and tilled dirt, as well as water. They can be roped through (note: dark water cannot be roped through, I think) and degrade at some sort of undefined seemingly random rate. If you put a nature tile next to a floor tile (or a floor tile on grass) the floor tile will degrade at a higher rate. My best guess is that is to make sure that an unused structure will eventually clear itself off the map, freeing up the space for someone else to build.
Putting a structural item such as a dummy, wall, animal gate, trash can, etc. on a tile next to a nature tile appears to keep the floor tile from degrading as quickly (compare a floor surrounded by animal gates to an ungated floor, the floor tiles will remain intact so long as the gates do).