Not it is not a fact. It's a pledge not a contract. However, they may hound you for a while. They get particularly pissed off if you seem to have made little or no effort to raise the money. In the case of VLM, there used to be a blacklist where charities could put your name if you were deemed to have abused a gold band place and other charities would be able to withdraw a gb place given to you if you were found on the list.
Contrary to popular belief, charities are trying to raise money for their cause, not provide an alternative means of entry to certain events.