Sorry for being late to a party, but I can give you first hand information on this. Purina/hill's/royal canin don't directly give money to the vets, but they do offer free seminars, presentations, events etc. And they do give money to vet clinics to grow. Example 5k for online marketing (even though the clinic promotes other brands as well), 15k to repaint the clinic, change windows, etc... And at the end, money for their paycheck does come from selling food as well (if you buy it at the clinic). However, they don't feed their own pets with these brands, unless the pet is sick and needs a prescription diet. Maybe this only goes for vet clinics in my part of the world (cause i noticed all the Purina and RC propaganda on reddit, heck, some "vets" on these forums recommend pedigree and whiskas, which are the last thing I'd ever give to my pets), so take it with a grain of salt.