Every scenario where someone identifies some class of accounts that are being "unused" inherently includes accounts that are being used by the owner in ways that are not visible from the outside, accounts that the owner intends to use but just hasn't gotten around to it yet, and a whole bunch of scenarios where someone registered the account for defensive purposes, such as preventing their stalker from maliciously impersonating them with the username they use on other sites, etc. Trying to sort out all those scenarios from "throwaway account that will legitimately never be used" is impossible without there being major collateral damage! It's far, far better for everyone for sites to have a bright-line policy of "once someone registers a username, it's theirs to keep until and unless they choose to delete it".