The DCO is not technically a license, but it is a legal document that usually comes along with a license from the contributor to the project (that license usually being the project license). It also serves to replace a Contributor License Agreement, offering only a base level of assurances that the contributor has permission to make the contributions. Without a DCO, a contribution is legally suspect; it carries no inherent guarantee that the contributor wrote the code or owns the code, for all you know the contribution was just copied off stack overflow, or from the contributor's company's private code base.
In practice, although it is not a license, the DCO should be accepted as is for the same reasons license text is accepted; free software does not depend on the freedom to modify the DCO, and is in fact better served by a non-modifiable DCO. There is still no real reason why the DCO itself needs to be licensed under free terms, and plenty of reasons why it shouldn't. This is a non-issue.
Regards,
Daniel J. Hakimi
B.S. Philosophy, RPI 2012
B.S. Computer Science, RPI 2012
J.D. Cardozo Law 2015