I imagine it is imposed on them. With physical books, there is a monetary outlay that places a limit on how many copies they can issue. This would have to be artificially imposed for e-books and the reason someone (the PLR?) would want to impose this is to protect income for authors.
Yes... it is a licencing issue. We have similar things with some bits of software -- the licence allows only (say) 25 simultaneous copies running on the central system. If you pay more you can have more simultaneous copies.
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