Sounds like common courtesy, which even works well in races. I mean, if I knew someone was trying to pass me on the side and was strong enough to just be moving up and calling it out, I'd give it to them just as I would hope they'd do the same for me. The roads can get tight and no one wants to crash out just because they didn't want to let a stronger rider pass. Plus, this isn't Mario Kart, we don't need to be shoving people off the track or dropping banana peels behind us to botch the competition.
There's a difference between blocking an attack and physically just being in the way.