Zulu>That's a good question. I think it's more because of habit than anything else. We're used to doing triples and it feels good, so we just think "triple" when we have to put a number on something.
But not everything in warm-ups are triples. Maybe I didn't make it clear in my warm-up program above, but the warm-up ends with a one, not at triple. The final set in my warm-up is 1 rep for 7/8 of the expected 1RM, that I'm about to do.
This is exactly because of the potential problem you point out. Namely, that doing triples with a weight close to your limit will definitely fatigue you. That's why I recommend a decrease in volume as the warm-up weight goes up. And when you get to your final warm-up set at almost 90% of your expected performance, you should do only one rep and then take a two minute break.
This is not an especially common way of warming up, and a lot of people has asked me if something was wrong, when I do only this one rep and then call another two minutes warning, but it works.