Are you involved with volunteer work along with your regular career? If so, you will know that there are similarities, but also differences, when operating in these two environments. One significant factor that plays a key and varying role is motivation.
Within career work, one could very quickly point to the obvious motivators such as payment, career progression, status, etc. Closer consideration might offer a positive work environment, the challenge and job satisfaction. However you look at these extrinsic and intrinsic drivers, there is an external orientation in terms of what you receive and just as important, there is consequence to not delivering. Leadership also has significant leverage in this situation which also creates accountability leading to pretty significant contributions.
With volunteer work, people do this for varying reasons and the resultant motivation leads to different levels of commitment. Additionally, in conflict with this is the constant that everybody is doing it for free which then becomes a source of frustration because of the disparity of contributions. Because leadership has minimal leverage in this instance there can be a lot of complaints and this can give rise to dissatisfaction.
The solution lies in avoiding comparisons. Simply accept that volunteers align their commitment / contribution to their own motivation and then make the necessary choices. For some this will be a lot of work and for others not so much. However rather than viewing this as an anomaly or unfair, simply view it from the perspective that people are making conscious decisions to contribute at their own level, and they feel good about it.