It really shouldn't be, though. Feral cats are a scourge.
Why? (serious question)
I know those statistics that an outdoor cat kills x amounts of birds, rodents etc.
Which is btw not well accepted at all:
http://en.wikipedia.org/wiki/Feral_cat#Birds
Loss's study and earlier related studies have inflated estimates of wildlife killed by cats in the U.S., based on unscientific research that extrapolates from tiny samples and projects them onto whole nations. One reviewer stated that Loss's study was filled with "numerous major flaws in the statistical arguments made" that in his view made it "unacceptable for publication". It was unclear how predation rates were obtained, and then "applying these estimates to all cats across the country is highly questionable."
However, I would be very surprised if any significant number of these animals are from other species than garden birds and rodents thriving around humans.
Feral cats don't go and live deep in the forest (and basically compete with wild cats), they stick to human settlements. That's their default state and how they came to be our pets in the first place. They have lived among us like this for thousands of years.
So they are only a threat to the same dozen species that also happen to be able to live in the artificial environments we've created; that's not really an invasive species in my book. They don't start killing rare birds/rodents in some nature reserve.
However, sticking to humans gives them an unnatural advantage of finding food even if they overhunt their prey and they can therefore still sustain a high population which, yeah, would threaten the existence of garden birds etc. High populations also make it easier for diseases to spread of course. Therefore all outdoor and most feral cats should be neutered (at least the male ones which is rather cheap and reduces noise from cat orgies

) to re-introduce some control.
But their mere existence? I just don't see what's so bad about it. If anything they can be used to keep the populations of "feral" rats, mice (and pigeons) in check - like they always have.
Oh and unlike feral dogs, they are also no actual threat to humans.
btw in Europe, where the general idea is that an outdoor cat is a happier cat and thus there are many free-roaming cats, we also see a decline in bird species:
http://www.exeter.ac.uk/news/featurednews/title_419684_en.html
While that study makes no mention of cats at all, it reflects what I mentioned above, that it's just the common species that are affected here.
The majority of the declines can be attributed to considerable losses from relatively few common birds, but not all common species are declining. Numbers of great tits, robins, blue tits and blackbirds were all shown to be increasing. Populations of rarer species, including marsh harriers, ravens, buzzards and stone curlews have also shown increases in recent years: this is likely to be the result of direct conservation action and legal protection in Europe.
Though I have to say this depends on the environment. As pointed out in the wiki link, introducing cats in habitats that don't really have small-ish top predators for small birds&rodents can be damaging (the biggest threat by far is still the loss of habitat by humans though). Australia is a good example (and various islands). In almost all other places you have animals like weasels which actually kill birds at higher rates than cats, thus the existing species there have evolved to deal with this. Not in Australia though.