Which is, frankly, part of the problem with our institutional view of the Reserve Force. If work is needed to ensure readiness- and pers admin fits that bill - then it should be compensated.
That's not a blank cheque. Pers admin should be, as much as is possible, conducted within the normal paid timeframes (trg or admin nights). but that's not always possible. What if clothing stores is only open during the day? What if the ID section won't work weekends? If someone is required to get a valid ID and can only do it during hours that the unit does not parade, then yes, that should be paid - it's a military duty being imposed upon them. (And should be authorized in advance, barring an exceptional situation. I know some units have standing authority in place for certain types of admin, such as ID cards or clothing stores runs).
It's lazy leadership to say "It's your problem, do it on your own time." (Of course, if Bloggins needs to make a clothing run every week, then there are other issues at play other than pay.)
There are larger issues as well - we now expect senior Reserve leaders to be responsive to emails, make calls/emails to subordinates, and respond outside of the parade/admin night nexus. How do we, how should we compensate them for those additional time demands?