The admins don't know user JeremyBernal from user RonJeremy, except by IP addresses, which is really only useful for finding ban evaders, spammers, and other ne'er-do-wells who make multiple accounts. The admins have started trying to get artists' accounts confirmed as artists through their FA or other art accounts, but unless the artist makes that effort, the account could belong to anybody. It could belong to the Pope for all the admins know. And if the Pope is Jeremy Bernal, then I'm Ron Jeremy and this is my favorite store on the Citadel.

So anyone could make a user account and name it JeremyBernal. That doesn't mean they are actually Jeremy Bernal. So if a user account named JeremyBernal decides to post Bernal's work, the admins would probably take it down, regardless of whether the account claimed to be Bernal. If it's not confirmed, the account name is worthless.

Artists can choose not to be on an imageboard for a lot of reasons, not all of them purely commercial. For instance, an artist may not want to be associated with pornography (a natural association given the overwhelming majority of content on this site). Or the artist may wish to control the channels of distributing their work because they don't understand the Internet or viral marketing. Or the artist may have a hate-boner for e621, itself. Or other reasons.

In the case of Bernal, with specificity, you must also understand that he has been somewhat "special", in that he has been especially threatening about having his artwork removed. Ditto Sexyfur. In fact SexyFur gets special mention on the DNP for disallowing promo materials. So if promo materials would normally be deleted along with everything else, special mention reiterating that policy should underscore how seriously the admins have to take the matter in order to avoid being sued.