I think it comes from the history of libpbs being distributed as an archive library, which means only necessary code was pulled into executables from libpbs.a and some of them might have needed crypto or libpthread, but not all.
Of course, now that it is a shared library, it would be way more beneficial for libpbs.so to itself record the dependencies on these two.
This topic was discussed in the PBS forums here: Compile OpenMPI with PBSpro 14.1.10
and a bug had been logged here: https://pbspro.atlassian.net/browse/PP-360
You are very welcome to submit a fix to this bug.
Thanks