I understand that the very native rationale behind why line staff moves (and is capable enough to do so) onto HR management is the obvious non-development of HR as a technique or a specific skill that can be performed only by HR professionals.
Eg. about Job evaluation methodology per se
One key finding of a research with WorldatWork and Loyola University of Chicago (of
more than 1,200 organizations) indicates that between 82% and 96% of organizations evaluate jobs, but only 18% proactively maintain their systems. Moreover, a majority reports that they believe approximately 20% of jobs are incorrectly placed within the job grading structure. As a result,we believe there is significant untapped potential to leverage job evaluation efforts to optimize organizational structures, develop people as key performers, and build employee commitment through reward programs that are fair, motivational, and competitive; so when this is the scenario pertaining to the most basic of our concerns then its pointless to deliberate onto the secondary endeavors!!
The bottom line is that the profession is still immature and only recruitment is the prime activity due to massive man power turnover being at its peak! Hence transitions from line to HR are easy and can run the show very very well. How many organisations that we know of religiously follow psychometric tests / assessment centers during recruitment? this further declines the specialisation that can be brought in by a HR professional and hence line managers are these days "nurturing" recruitment directly.
As far as the reverse is concerned, i strongly feel moving from HR support to line is a difficult transition, unless you are equipped with thorough professional training in the desired line field.