That is the point.
This is a fake by-election called not because the candidate wishes to resign, has been charged with an offence, is ill or has died, or any of the other valid reasons. It is a political scam of Farage v the "Establishment" to feed the significant paranoid base who believe the world is controlled by a mystical elite cabal.
Had he waited for the Standards Committee to report and, if found guilty of committing a breach, resigned and stood, I would support your view.
However, he has protested his innocence but then acted as if guilty. He has voluntarily put on his fluorescent jacket and joined the ranks of those picking litter for community service before any court has ordered it.
Why should the other parties feed this political victim-building stunt?
And what makes this all the odder is I suspect his protestation of innocence is well made. He has been silly but not dishonest. Had he cooperated fully with the Committee, received vindication and let this blow over, it would have been chip wrap years before the next general election.