I don't think there's a big profit margin on the stuff - I kind of assume they're paid to stock it. Paid for the placement, same as a supermarket or bookstore.
I can tell you for a fact they're not paid to stock it. The company does offer some benefits to vets, but they're not really that awesome.
1) A 100% return policy. They'd take back any product for any reason, opened or not, and give a full refund, which meant the vet could also give clients a full refund. This was most commonly used when terminally ill animals passed away. The owners could return that half-used bag of food for a full refund and put that money toward cremation expenses or whatever else they needed.
2) Good shipping fees. Any order over 50 lbs earned free shipping. Some of the other companies aren't as generous, and dog food is heavy/expensive to ship.
3) It's widespread. We've had people come into town on vacation only to run out of their pet's prescription diet. They'd come into our clinic and we could, at the request of their normal vet, fill the prescription. This benefit was particularly useful in the aftermath of Hurricane Katrina, when a lot of people's normal vets were completely shut down and owners had evacuated with their pets all over the country. The ability to find your normal dog food is a big deal for animals with kidney or heart disease, and a similar product from a different brand would still carry the usual risks of stomach upset that can occur with dietary changes.
That's pretty much it. Useful things, certainly, but not the be-all/end-all. There aren't kickbacks, the food isn't sold on "commission" as one person stated. It's a product, not the greatest product in the world, but the best option at the moment when you consider the range of factors a vet has to take into account. The minute a better quality product that offers the same range of prescription diets at a better price comes on the market, I guarantee vets would switch over in a heartbeat.
I don't think saying 'here's this brand of food that MANY vets offices stock, though it's made of crap,' is the same as 'use proper flea products or 'vaccinate.' There's a reason to be suspicious of the former, imo.
See, to me it IS the same kind of mentality. The people who have problems with *insert flea product here* say, "Vets just want me to put poisons on my animal. They get a lots of money for the flea stuff and then they get more money when the poison gives my pet kidney disease and I have to pay for treatment. They don't care about animals at all. Anyway, everyone knows dish soap kills fleas just fine. If vets were honest, they'd just admit it instead of trying to sell me this overpriced stuff. The whole thing is just fishy. Since most vets push this stuff, they're clearly in the pocket of the manufacturers."
Frankly, when someone comes into your workplace and accuses you of such things, it's offensive. It's an undeserved attack on the character of people who are doing their best for their clients and patients.
Like I said. Science Diet isn't necessarily the most wonderful food in existence. However, that doesn't make it BAD food. That's like saying a salad isn't healthy because it's not made with organic lettuce.
Our vet sold Science Diet, but he also recommended a couple of other brands, and there are so many fly by night companies out there that he couldn't keep up with all of them. His mentality was, "This is what I sell, and it's a solid product. If you'd prefer something else, go for it. As long as the animal is healthy, I don't really care what you feed it."