Boo to leg extensions. I'm not sure there's a connection to ACL injuries, but it's not a great exercise in my book.
In fact, I'll suggest that it's probably not contributing to ACL injuries, and may be a good rehab tool post ACL tear. But if you are not rehabbing from knee surgery, then I'd suggest you shouldn't be doing leg extensions.
I have an underlying dislike of single-use exercise machines. Here's the thing: they require that your body moves exactly in one dimension, without variance. But your body doesn't actually move that way. There are variances. It happens. But when you use a machine, it can't happen. And the result is that there will be imposed stress elsewhere because the machine doesn't let your body move the way it's supposed to move.
My second reason for disliking leg extensions is that it's getting you to build strength in an open chain leg extension movement. Open chain basically means that the end of the limb in motion is not attached to solid ground (example - bicep curl is open chain for the arm; squat is closed chain for the legs - legs are stuck to the ground). So why are we adding 10s to 100s of pounds of weight to a movement that the human body may not be intended to do? We lift, and drag and pull, and push, but effectively giving a slow kick to a huge pile of weight? Really?
Even in a rehab setting, I think the leg extension has very minimal use, and once you get even close to heavy, you should be doing closed chain work with the legs, not open.
Hopefully that sounds reasonable.