I read this somewhere earlier today. The reason why most people spend a lot of money on entertainment is because they have too much free time in their hands and since they don't want to get bored they'll pay for entertainment: be it subscribing for cable TV, going to the movies, etc. So that's where volunteering comes in. If you are doing something in you free time, you won't have any free time and that means you won't have any time for entertainment and consequently you'd spend $0 for entertainment. It sounds like a Neat way to save money. Would you volunteer [to help your community] so that you can save entertainment cash?
I think people should volunteer to help others primarily and they save the charity or cause money. I can see what you mean, rather than find things to do that cost you can spend your time doing something useful for free, yet it is not always free. My parents are retired and work in goodwill shops and they pay their travel expenses to get there and some volunteers pay for a taxi to get to work. My mother buys tea and coffee for the shop, so she doesn't really save money by working there, but she likes to help. I have volunteered, but not to save my money but to save the charity money.
The point of volunteering is supposed to be to help a good cause, but I agree that in certain situations it can save you money. I once volunteered in a small thrift shop that gave its volunteers employee discounts on the store items for volunteering. So I was able to buy a used TV for my apartment at a discount, which helped me save because I was a broke 20 year old at the time.