Habitat destruction is a much more understood thing in these days than it once was. Yes, it's a big reason for long-term songbird population drops in some areas/species over the past 20 years. Protecting existing habitat is important and we have some damn fine groups and agencies doing that work in a responsible manner-and yes, they could do better, but that would mean more public cost to do so.
Feral cat kills, however, happen in urban/suburban/exurban areas, where designated wildlife areas like parks, greenways provide suitable habitat for the birds. Often times extraordinary measures like birdbox trails are in place to assist cavity nesting songbirds (such as the beloved Eastern Bluebird, in Wisconson's case) by providing the nesting habitat that is no longer readily available. In rural areas, it's all about the sheer scale of feral cat colonies-hundreds of cats are going to need to eat lot of stuff. And that means birds, mammals, reptiles, whatever they can eat.
So you go through all that work-setting aside parks and greenareas, building and monitoring birdboxes (tossing out starlings and house sparrows), etc, and then some feral cat comes and eats the songbirds. Bleh. Or entire rural areas are depopulated of small wildlife due to feral cat colonies (remember, feral cats are an INVASIVE PREDATORY SPECIES on ecosystems). Double bleh.
Let me stress that remark-feral cats, when introduced to an ecosystem, are an INVASIVE SPECIES. If you look at it that way, you can see why just shooting them has some merit in law-invasive species are generally considered detrimental to ecosystems, especially ones that introduce such remarkable predators as feral cats do. They are also systematically eliminated.
Again, I want to say that there has to be a better way. I'd say that the issue should be one of county and municipal law, though-a rural county might want to give that privlidge to landowners with feral cats on their property. Muncipalities might want to take a more "humane" approach through systemic spaying/neutering and accelerated termination of strays.