These people never bothered me.
The other day I was driving around my new hometown looking for a grocery store and I spotted a small group of people accosting people at an intersection looking for donation. Because I was driving around a bit, I ended up going through the intersection three times, so I had a lot of chance to observe them. They struck me as more aggressive than the firefighters I remembered from PA. I didn't have any money on me, so I just shook my head at them when they approached my car. To their credit, they did move on, though, of course, I had to do this three times. It was only on the second time through the intersection, though, that I noticed the labels they had taped to their buckets... apparently one of the local Christian churches.
Of course, the truth of the matter is given the climate in this country, I'm not really sure that donating to firefighters in the street is really any better than donating directly to a Christian church. You can't really question the guys on the street corner about what exactly they are doing with the money, because they often don't know. And if they don't know, the firefighters are as likely to be collecting for some church as for anything else.
For the average American, who subscribes to the Christian/majority belief system, they don't really care. But as an atheist, I can't in good conscience give my money to support beliefs which I find to be harmful and dangerous.
However, what I will do, is add a couple dollars to my personal charity pool, the one I'm saving for a good secular charity this year.