Pet or not it's nature in all it's harshness. Cats did not start out as pets and had they never been pets they'd still be out there breeding and killing just as dogs and every other animal that has been domesticated would be.
I'm willing to bet that the fact that cats have been domesticated has greatly reduced their numbers through spay & neuter programs.
At my bird feeder not only are the birds eating seed but there are rodents eating fallen seed, opposum and skunk eating rodents, cats eating rodents and birds and owls and hawks eating all of them. Mother nature pulls no punches.