In practice, I have seen the following common scenarios.

* Companies and businesses, most often have a paid account with GitHub. This allows them use Github with the code which is not yet open source or will never be open source. It is simply using Github.com as an Infrastructure as a   Service, which many companies will willingly do. If it is using this paid account is unsuitable for contributing to open source projects then a free account could be created by the contributor.

* If the company encourages to use to free account, then I think, the company realizes that the person may contribute to, not just it's open source projects, but to other open source projects hosted in Github using that account and should be open to that.

* A third scenario was mentioned by M.-A. Lemburg, wherein the company uses free account, but the company or the individual, more likely the latter, for some reason will not like to use the same account or other project contribution. In that situation, the choice is to buy a paid account.