It's been a long while since I read it, but the Commonwealth forces did not especially welcome the Dragoons, and treated them like any other Mercenary group. Badly. The company Store ploy was common in all the Great Houses, but Steiner Commanders added the extra twist of depleting the Dragoons with missions causing Heavy Attrition. Then Kurita made an offer that was very appealing and the Dragoons accepted the Contract. With the usual disclaimer that they not fight the previous employer.
Of course, the Combine had their own ideas of how to get their hands on the fabulous equipment the Dragoons had.