Freely given???

Whilst walking down chelmsford high street with my friend Fran we past a group of people that were working for a charity,they were stopping people as they walked past. Now we both spotted them, avoided eye contact and quickened up the pace however we were targetted. After being cornered we had no hope of passing and stopped. Now this was a charity to help raise funds to buy a wheel. The man asked us what we were doing and where we were going. We said we were heading for starbucks and then later to the cinema. We were then asked if we would like to buy a leaflet which cost 3 pounds and the money went towards the buying of a wheelchair. Now to be honest neither of us wanted to give them the money so we said we wouldn't like to buy it. Now the man reminded us what we were about to do, told us that we should get are priorities in order and in a very subtle way told us to stop being selfish. So we did, we brought the leaflet and walked away. However both my friend and I walked away feeling slightly annoyed and bitter.
This reminded me of a time when i was sitting in my flat when five boys walked into my room, shut the door and said they were raising money for a charity. i do not no if they had Id or what the chariety was, i just gave them some money so they would get out of my room.
So my point is do charities persuade, bribe or guilt people to give them money and is this right? Should it be the charities job to think about the giver as well as the cause and is the reason we walk past people raising money on the street is because we do not like being put under pressure or being made to feel gulity?