Ah - it's called marketing and sponsorship. These folks to do not approach the companies - the companies come to them. Same as any other althlete or celebrity. Paid endorsements.
The thing is, I have more respect for a trainer or rider who has proven themselves in their field and is asked to represent the company (like Ariat, or Wrangler, or Professional's Choice) than a clinician who slaps their own name on a bunch of training gadgets to be sold exclusively through their "training program" just to make extra bucks off their loyal followers.
Trevor Brazile is a great all-around cowboy whether or not he is endorsing Wrangler. The PBR riders who endorse Ariat have proven themselves in their sport with or without the products they help to advertise. Karen O'Connor, Beezie Madden, George Morris. . .they're all extremely talented horse-people who have proven their talent and gotten where they are by being GOOD at what they do. Not by relying on starry-eyed followers to buy their over-priced "stuff."