We do the same when we do certain sweeps. Holding the person and controlling the fall is the only way to keep a training partner from being hurt. We also do a riskier form of training where we sweep with the understanding that our partner will be able to avoid the sweep. This allows us to train our long range sweeps. The rule for that type of sweep is, if it seems that your partner isn't going to be able to avoid the sweep in time, stop the sweep.
In competition, I wouldn't want to make a rule where I would have to control my opponents fall. I would rather just make it illegal. The reason why is because depending on the type of sweep that is done, there is no opportunity for me to control anything but the force of the sweep. In other cases me trying to control my opponents fall may put me at risk of my opponent taking advantage of me.
Here's an example, I was training a female student (who is a hyper teen) with how to do sweeps, so I performed a sweep on her. In her mind she felt like she could defend against the sweep (aka, the sweep doesn't work). So when I swept her, I held her up and she started kicking wildly, with kicks that landed near the groin while I was trying to control her fall. She had no concept of when the technique ended and me trying to protect her from falling on the ground started. So I'm trying to keep her from falling on the ground and getting injured while avoiding groin kicks. I can see the same thing happening or something similar happening with competition to the rules where a sweep would be legal provided that the person controls their opponents fall.
In training, all of the sweeps can be trained safely by using ways to either control the fall or use another tool to simulate a leg. We use a swim noodle to simulate a leg which allows us to do full force sweeps without any risk to our partner.
