It's basically squatter's rights. Tapas wants first crack at anything that could potentially yield large profits.
We can partially understand where they are coming from. For example, if Tapas saw a new series pop up on their site, such as One Punch Man 2.0 and they started heavily promoting it, and it took off and became a huge hit, then yeah, Tapas should get first right of refusal, that's only fair. After all they put forth a lot of effort and resources in helping the series succeed.
However we don't agree with these rights if the creator alone does all the legwork, spent all the marketing money out of their own pocket, really built up a huge following all on their own and then Tapas rides in to reap the harvest when a third party comes around and offers some serious cash for a series.
In that situation a creator could just delete their account and then proceed with negotiations. Tapas couldn't do much to enforce these rules other than subpoena the creator's email records to see when the offer was first made and take it all to court. Given the company's size, can't see them using up all of their time and resources fighting a legal battle over a webcomic, unless it becomes a multi-million dollar franchise, which seems highly unlikely.
Another potential work around is to simply submit the series as a pitch to the submisions@tapas.co address before starting to post. Most likely a creator is going to get rejected. Then later they can use that email to prove that Tapas gave up their first rights already.