Her leaving the party is totally understandable, she is not a good fit for Labour and hasn't been for a long time. They've treated her badly, and everything in her letter is basically true.
However, if she was resigning, as she claims, because her conscience cannot allow her to remain in a party where people are claiming expensive gifts (which are within the rules) while implementing policies that harm the poor, she cannot hide behind the rules that say she doesn't have to trigger a by-election.
If she acts according to her moral conscience, she should be 'better' than the rules and allow her constituents to decide whether they still want Rosie, given that she only stood weeks ago as a Labour candidate and was arguably elected on that basis.
She chose, instead, to pretend that her constituents didn't elect her on that basis, but for her 'independent-mindedness'.
She is now saying that she will keep her £93k a year job on that pretence.
If we are talking about money-grabbing, that amount of money has to come into consideration particularly if it could be viewed as an unearned gift.
If it is not a pretence, then she should be willing to call a by-election in the knowledge that her lucrative job is safe.