Don wasn't kicked out of the Fergus Stables. He wasn't even suspended. He left Karl Walton's office as if nothing had happened. Almost. In fact, much has happened.
First of all, no one mentioned that Don attacked Martin. This topic was passed over in silence, which meant that both Martin and the other drivers decided to put these incidents into oblivion.
But Alston knew that nothing had really been forgotten, and if he made even the slightest mistake, he was likely to be killed or beaten so badly that he would become crippled. Steve was unpredictable, and perhaps only Martin held back his aggression, and when Paxton got mad, his punches had terrifying power and speed. Don was no coward and he could fight, but against Paxton he had no chance. One false move on Don's part, and he would be happy if his adventure ended with the fitting of a denture.