I think it is:
I think most cases of posting about race A in the thread for race B are either:
1. Those situations where people get stunned that the same team gets multiple wins in a day, and makes a post like "Oh, wow! Trek got 10 wins in the same day!" in the thread for the latest-finishing race. Now, I agree that the thread for that particular team would be the correct place to post about such matters, but I can understand if someone might get carried away.
(Also, I suppose if a team managed to get 10 wins in a day, it would probably be during the big National Championship Sunday, so... that wouldn't be a problem.)
2. People getting threads mixed up, and for example does stuff like posting about Milano-San Remo in the Milano-Torino thread.
As for caring about it:
Well, I don't care about reading the result of race A in the thread for race B; either I have already watched race A, or I won't.
I will of course take care not to post about the result of race A in the thread for race B.