I agree that subscription managers should be able to create users. I've created a Bugzilla entry for this at http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=2716
With regard to restricting subscription managers to particular subscription types, that's more specific to your situation, so unless we get a lot of requests for this feature, we're not likely to implement it. However, if you do write a patch or plugin to accommodate this, consider posting it here in case another user finds it valuable.
Open Journal Systems Team
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