Certainly, Abaqus could license documentation of their file format to prohibit details from being disclosed to people who have not agreed to the license (which would prohibit open source readers).
A license would not be required if someone was to reverse engineer the file format. However, in order to do the reverse engineering one would need sample odb files. It is possible the Abaqus license prohibits sharing odb files with non-license-holders in an attempt to prohibit reverse engineering. I would be surprised if this were the case (it might not even be legal to enforce this license everywhere), but it is important not to induce people to violate agreements they have made.