I did not read all of the responses so forgive me if this has been covered.
Are these racing TB colts?
Is the bit a Chifney Ring Bit made for leading? Is it snapped into a halter?
If so, it is proper to train TB yearlings to lead in them and to respond to them. The only problem is that they need to be taught to lead by someone strong enough to hang on to one that wants to bolt. If a person is not strong enough to hold a bolter, then they should not be leading it as it will get very spoiled.
Years ago, I used to fit quite a few of these sale horses. They all had to be taught to lead correctly with a ring bit in a leather halter. I really hated to have to deal with one that had already learned to bolt and get away. [The only thing worse is a mule that has learned that same trick.]
If you have one that learned this little trick, you are going to have to ask the boss or another big strong person to let this horse try it with them and they have to be able to jerk this horse around and hang on. Otherwise, he will just get more and more spoiled.