The majority of members do not volunteer (this used to be a requirement to join - no pay your way out of it) and of all of the types of volunteers we get there is the least opting for "office duty" so pretty much everyone automatically gets the $50 charge. There is a list from work day for everyone who showed up or you can get a director to vouch that you put in your eight - let them know in the office and off it comes (or send a note with payment).
Those that volunteer almost always put in more than the eight so it's not really just the money. For the rest, $50.00 divided by 8 = $6.25 an hour, perhaps they look at it as a (in)convienance fee.