The Center for Global Development Global Health Policy Blog explains very clearly why they are in favor of “positive price” bednets (meaning that the end user has to pay for them) rather than distributing these important anti-malaria tools for free. They also support positive pricing through a combination of public and private distribution chains. According to most studies so far, these methods are more sustainable and more effective means of getting enough bednets into a population.
Although the CGD lists a lot of arguments to support their position, I noticed that they chose not to mention one that I’ve heard often. Namely, that people are more likely to value (and therefore use) something that they have paid for. I don't know if there is evidence to support that claim, but it comes up reasonably often.
There is a lot of lively debate on this issue, which is obviously relevant for a range of commodities that goes well beyond bednets.