I agree that there are much better ways to give. A smaller organization will probably be more efficient; 20% of your donation is likely to be overhead if it goes in the kettle.
They try to guilt people into giving: "look at me standing out here in the cold ringing this bell all day, if you walk by me your a horrible person"... "I mean happy holidays!"
They tap a resource to feed the poor, one that would go to waste.
The intent of
Matthew 6:1-4 is not to hinder you from giving. It is a warning against giving with the intent of having people see you doing it. People do this to gain some social advantage. If your giving to a stranger that doesn't know exactly how much your giving there is no social benefit for you.
How would this situation be any different than putting money in a collection plate?