The free-rider problem is familiar in economics. It means taking advantage of a product or service without providing any return to the provider of that product or service. All open source users start out as free-riders. They download and try the software, and perhaps deploy it, and do not generally consider contributing to that software's development until they are already using it and desire an additional feature.
If they desire an additional feature, they may implement that feature themselves rather than pay one of the initial developers. At this point, they are not free riders. Businesses that join an open source project as developers contribute some software to the product, and all of those businesses derive an economic benefit from making use of the software in a cost-center of their business. There are developers that are not motivated by the desire to provide software for a business cost-center. These are individuals whose motivations are primarily artistic, and scientific researchers.
Volunteers derive emotional fulfillment from having users for their software, just as artists derive fulfillment from having others appreciate their paintings. For volunteers, users provide an intangible benefit which the volunteer desires. Thus, those users should not be considered free-riders.
Companies that place importance in a particular open source product tend to hire developers who have already gained stature as a developer of that product. Thus, individuals who have started with no pecuniary interest in the open source project tend to find employment with an organization that does have such an interest. And thus individuals who participate in open source development often reap an economic gain from that participation. This is another reason that users should not be considered free-riders by these individuals.