This is science fiction, not the real world. The background of Dickson's stories assumes that the human race has split into splinter factions, with each faction specializing in a particular area of expertise. So the Dorsai need to take along their own combat support, since their employers won't be able to supply such.

Also, the Dorsai would train with so many fire teams (for example) to handle such and such anticipated opposition. If an employer didn't want to hire enough Dorsai to handle the opposition according to Dorsai doctrine, then the contract simply isn't completed.