I think that it might be a "simple" question about the function of bptm vs bpdm.
1) bptm: nbu tape manger, but why it actually does work for both all tape-based and most disk-based policy jobs？
2) bpdm: nbu disk manager, so why it only works for a few disk-based policy jobs, such as deletion or duplication？
Is there a specific problem / issue that you are trying to resolve ?
My guess is that the question of "Within which daemon / service / process should any functionality reside?" is probably a function of ease of development / debug / support.
I have noticed roundabout NBU 6.x that bptm also started to log disk backups.
The 'nice' thing about Activity Monitor, is that we can see in Job Details where bptm and bpdm processes were used.
If you need a reason, you will need to ask the developers...