I would disagree 100% that there was anything disrespectful associated with the race. There is an official guide assigned to our group from the Angkor Archaeological Park who guides us and oversees everything. Most of the racing occurs outside between the temples. The scavenger hunt inside the temples is largely to find other official Archaeological Park guides to ask them specific questions on the history and cultural relevance of temples.
I've found ABD highly respectful to all people, cultures, and places. That's quite possibly the last criticism I'd ever expect to hear regarding ABD. I'm sure the same can be said for A&K,
Nat Geo, Tauck, etc. If my discreptions have given the impression otherwise, that probably has more to do with me not adequately painting the right picture rather than any cultural disrespect of ABD.
I certainly don't want to convince you ABD is THE way to see SE Asia. It was wonderful and worked for us, but as you said there are many good options. However, I wouldn't rule it out because of the scavenger hunt/race. I think if you were to see it in person, you'd have a different impression of it.