I'd say that's a much bigger piece than the "well they're not tackling at all crowd."
They are contact restricts across all levels of football, especially live situations. I don't know what it is collegiately, But I'm sure it's similar. And coach's don't want to go to the ground anyways. That's when the majority of your injuries occur, limit it as much as possible.
Pretty much every program I'm guessing runs the majority of their practices at Thud pace. It's high and hard. Some guys do get tackled to the ground, but you don't have dudes landing on top of each other. That's the only difference.
My guess is this...They're Thud, but the whole mouthpiece thing does hold a lot of credit IMO. If you're not worried about getting your shit rocked, because you've got quick whistles...than, that makes sense. And if you've got quick whistles, dudes are pulling up and not really playing at game speed.
That's the problem IMO. If that's what's happening.