Unfortunately, I have to agree with you. Yosemite is not the only park we have seen people feeding the animals. Once when we were in the Rocky Mountains, there was a family feeding some squirrels near me and I nicely said to the father that it is against the park rules and it is harmful to the animals to feed them and also his children could be bitten. The man started screaming at me that he can do whatever he wants to do and approached me in a aggressive manner so I walked away. Nearby there were some NPS volunteers and I told them what happened and they started walking towards the man and when he saw them coming, he and his family jumped into the car and drove away. That was the only time that I have had that kind of reaction. Usually when I have asked people not to feed that animals and explain why, they stop or a few just ignore me. I started carrying Leave No Trace educational cards with me and hand them out to people who are interested.
But even though the parks have things in their guides, brochures and signs, some people will always believe that they are above the rules.