My dog, along with most UK dogs, is not left alone to free roam outside unsupervised. He is walked on a lead and only goes out in our fully contained and secure garden. Dog owners absolutely are more responsible. You cannot, in any meaningful way, compare the activities of my dog, who has never killed or hurt any animal (and we have a lot of wildlife in our garden, including hedgehogs, frogs, foxes and many garden birds as well as our pet guinea pigs) to a completely unsupervised cat, who have left fog entrails strewn across my lawn for my kids to find. They are not the same. One lives with me, completely supervised, one buggers off alone for hours on end and their owners have no idea where they are and what they are killing. You're making no sense in comparing the two, especially since there is absolutely no evidence or available statistics whatsoever that dogs are a problem for wildlife.
As for the RSPB, they are widely recognised, in many circles, to be simply pandering to their cat-owning members, and pussy-footing (pun not intended) around the issue so they don't offend the cat owners and lose donations. They know full well the effect cats have on birds and other wildlife and they know the numbers killed are unacceptable, because they do condemn bird shooting sports, which kill far less birds than cats do (100,000 per annum as oppose to the millions per annum killed by cats.) Their website states: we believe it is now time to regulate the gamebird shooting industry. But not regulate cats, who do more damage? Very telling.
They even sell cat deterrents on their website! They just refuse to come out and say it.
We're not conveniently ignoring anything. We simply don't hold them in esteem due to their hypocrisy and cowardness.
But aside from the RSPB, the International Union for Conservation (they've classified domestic cats as an invasive species) do condemn outdoor cats, as do the countryside alliance. The RSPCA and even the cat's protection league do advocate for "responsible cat ownership" and it's hard to see how letting a cat free-roam with no idea where it is, can be seen as responsible.
The culture in the UK is currently completely in the cats favour, with most people caring about the cat's wants and freedom than the local wildlife population, hence why you don't see much public condemnation of it by UK-based organisations, but it's quietly and very slowly starting to come around to the majority of the world's way of thinking, with more and more people seeing the light. Hopefully some more charities will grow some balls and start condemning it with us and we might get somewhere! But it would appear we need public opinion to sway first.