My take is that if both cars are running the full season, then they would both be competing for a Championship. So therefore they should both be running heads-up when facing each other. May the best man and car win. But if say Billy just races a few times during the Countdown as a blocker, then one might question if they don't appear to race heads-up against each other.
By the way, I don't have an objection to having a car as a blocker when needed, or even "taking a dive" if needed when racing a team during the Countdown. What the heck is the point of having your own team car take you out of the Championship hunt? It makes no sense. But we all have pretty strong opinions on that topic.