For more information, see the GNU Coding Standards:

https://www.gnu.org/prep/standards/standards.html#References

In particular:

> A GNU program should not recommend, promote, or grant legitimacy to the use of any non-free program. Proprietary software is a social and ethical problem, and our aim is to put an end to that problem. We can’t stop some people from writing proprietary programs, or stop other people from using them, but we can and should refuse to advertise them to new potential customers, or to give the public the idea that their existence is ethical.

Excluding material isn't censorship---GNU stands by its principles. You wouldn't expect a vegan organization to advertise means of obtaining meat, for example.