Well, I've been a volunteer. Yes, it is permissible for people to question things.
But when you are trying to plan a dinner, say, for all the volunteers at your parish, and you are volunteering to plan said dinner, it is indeed frustrating when all you hear are questions and complaints. Frankly, most of the questions are really complaints.
The day and time of the dinner are questioned, the menu is questioned, the location is questioned. All by multiple people, multiple times. It doesn't matter if you choose paper plates for ease of clean-up or china dishes to save the environment--that choice, and you have to make that choice at some point--will be questioned. You can have 4 different entree choices, taking into consideration vegans, vegetarians, the most common medical conditions and allergies and still people will tell you that you have made all the wrong choices.
They will nit-pick the difference between boiled potatoes and baked potatoes, green beans and peas, thin and thick crust pizza. "Oh, why are you having peas? I hate peas!" when there are three other vegetables to choose from, plus salad, plus four entrees, two choices of potatoes, rice, pasta and rolls. No one is going home hungry, even if they hate peas.
It is amazing how otherwise nice and kind people turn into total nit-pickers the second there is a free dinner for them. At some point, when you are planing an event for 100 people, not everyone is going to get their first choice for date, time, location, menu and ecological soundness. And from my experience, you do not get compliments, only complaints.
A serious complaint, like there are peanuts in 75% of the dishes being served? Yes, that deserves consideration. A whine about how we should start an hour earlier so that everyone can get home on time to catch Survivor? Does not deserve the same degree of concern.
Questions are fine. Questions that are really thinly disguised complaints are not. Maybe the answer is for the questions and complaints to be raised only with the paid employees and not the volunteers.
And it would be nice, once in a while, to hear a "Thank you" instead of "Why on earth are you doing X?"