A lot of the things we sign our kids up for, from scouting to sports, require volunteers to make them work. No volunteers, no youth programs.
Making time commitments is no small favor to ask from people. Volunteers have to sign up for things weeks and even months in advance, and sometimes make major time commitments. Parents have all kinds of things competing for their time, tend to be overly optimistic about how much time things will take (travel & transportation included), and take routine household things like laundry and dinner for granted. Any parent who volunteers makes no small sacrifice.
As much as it's a commitment because of invasiveness and inconvenience factors to the individual volunteer, a volunteer organization is dependent on its volunteers to honor their commitment. Events and activities don't happen without the volunteers to make it so.
We remember those who have a gift for it - the brilliant coaches, the organized administrators - and these are the people who remind us that volunteer organizations are capable of amazing things. There are volunteers who do a crap job of it: they show up but make a hash of things. Still, there is a lot to be said for any volunteer who follows through on the commitments they made. In a lot of youth organizations, that's all that's being asked. Just show up like you said you would. And if you can't - hey, things come up - let those who are depending on you know that you can't. Withdraw if you must, but withdraw responsibly.
Your commitment is your obligation. Volunteering is optional. Commitments are not.