I've said it before, and I'll say it again. That's just arrogant, and misunderstands most religious people. Those few who are good just because they are terrified aren't really being good, to be sure -- but that represents such a tiny minority of religious people as to be a terrible point. Most people, even religious people, who are good, are good because it feels nice to be good to others. Not for any cynical reasons.
It's arrogant to assume that somehow you are "better than them" because you are an atheist and they are not. In point of fact, most charities are still set up by Christians/ other faiths, and there tends to be a greater number of people working in Charity who are religious. If that's for cynical motives, it's a) rare anyway, and b) hardly as important as the work they do.
A more important question should be why does it seem to be that people who are religious tend to be more likely to give to charity than atheists? Or, why should it need some promise of eternal paradise to motivate people to do good work?
Anyway, it's a false claim to imply that religious people are "less good" than atheists. False, cynical, arrogant and yes, even insulting.