You have to question the fundamental basis of a union if they're not even prepared to rally together to protect their own reps. All this talk of solidarity, when you have outright bullying/scandalous behaviour by Royal Mail and they didn't think the membership would support further action? That couldn't be more wrong, if they truly thought that then they massively underestimated the membership and their resolve.
I think what's more likely is Royal Mail threatened the union's existence by removing payroll subs and ending paid release for reps, both were being uttered by Simon Thompson, we all know the devastating effect this would have and there's no real counter threat the union could offer so had to make the best of a bad situation.