From their terms of service:

They (private repos) are also allowed for really small & personal stuff like your journal, config files, ideas or notes, but explicitly not as a personal cloud or media storage.

I’d guess that most private git repositories are small enough to fall under this category (unless you track large non-text files in git). This also seems like a very reasonable policy, considering that they’re a non-profit and they want to focus on supporting open source projects.