[boost] IOstreams: Why are mapped_file devices not seekable?

In both of the implementations of mapped_file_source and mapped_file_sink, the devices are not seekable (whereas basic_array_source and basic_array_sink are). I am wondering whether there is a rationale for this, or whether it was simply an oversight in development. Since mapped_files are still contiguous piece of memory I see no reason why they should not be seekable. On a side note, should a direct_tag not imply ssekable_device_tag i.e., struct direct_tag : seekable_device_tag {};