I don't particularly think that people who can't afford to take care of themselves ought to be having pets at all. Pets deserve homes where they have access to shelter, food, medical care, and such. Of course, this is an ideal world, and I suppose it's a little different if the only alternative is putting your pet on the streets, which would really be the worst thing.
I'm also vegetarian, and my cat is obviously not, so it's not really a simple matter of giving him the food I otherwise would eat. If there was nowhere for me to rehome my cat - have friends, family, even a shelter care for him as I can not, then yeah, I'd probably spend less money on my own food to take care of my cat. But honestly if I were literally living on the streets and couldn't get enough spare change together for my own food, then the pet most likely wouldn't want to stick around with me anyway.