People doing a job for which they are employed can indeed claim expenses from their employers, and people carrying out voluntary work can in some cases be paid expenses by the organisation for whom they are doing the work, and people paying expenses for voluntary work out of their own pockets might in some cases be entitled to set this against income tax. All of that applies to "politicians" as much as to anyone else.

The question is, why people who were formerly employes by the state should be entitled to be paid expenses by the state for things they do after they have ceased to be employees.