I can't speak for anyone else in the thread, but as for me, there's a huge difference between (1) having the government take money from me in the form of taxes1 and use it for what I would consider charitable pursuits, on one hand, verses (2) me giving directly to charities, on the other.2 If I am in control of my own giving, I can give where I see the need and am reasonably confident my money is going to a cause I feel should be supported because it is doing good work. If the government does my giving for me, I have no such confidence. Additionally, the government tends to be incredibly wasteful and inefficient for many, many reasons, not the least of which is that, as a body, it is not in the business of charity and has no idea what it is doing. So, yes, there is a huge difference in my mind.
1. For the record, I am not against taxes, in principle. I believe the governement should tax us as little and as reasonably as possible and should use tax money responsible, but that even if it doesn't do either of those, we are obligated as citizens to pay our taxes regardless.
2. Or, better still, that we give directly to those who need it, since a lot of charities themselves often have huge bureaucracies that waste a good portion of money that is donated.