You're right, I'm not for a theocracy. Once again, forcing a mitzvah cheapens it.
However, just because a value can be found in all religions/cultures, doesn't mean that everyone actually does it. I don't know what to say, about how to actually get people to do it, but I'm not opposed to using taxation to accomplish this ideal that exists in all cultures present.
The atheists, Christians, Buddhists, Hindus... They all agree that helping the poor is good, but do they actually do it? Ideally, a huge educational campaign on the universal values of giving and kindness would be best, but I don't trust it to be effective enough.
Maybe those who don't sign on to donate their money will get their businesses boycotted. I don't know. That's one way to enforce this without your "violence" of taxation.