"It is part of the business." "It commonly happens." "You will get experience". But you asked if it was fair. So, although many supervisors and principle investigators here are saying yes it is fair, I would say it really depends...
Unfortunately, the priorities of the supervisor do not necessarily intersect or coincide with your interests or priorities. The more important question is: Should such administrative work have your priority at this moment?
How will it affect you planning, other tasks, and responsibilities?
Did the supervisor tell you in advance that this was going to be part
of the job?
Are you pursuing to the goal of staying in academia and
writing grants later?
Do you want to stay longer in that research
group, do they even offer you to stay longer?
Will you be involved in
a feedback loop of the proposal to learn, or are you just used as a
On the other hand, what would be the consequences of saying no? (if it is even possible to say no)
Yes you will learn from writing a grant proposal, but it also takes time and energy. It would be rational to allocate your limited resources wisely. And for that, you need to consider your particular situation and the possible consequences, the choices, and priorities you have.
Because gaining experience in writing grant proposals for others will not necessarily be a wise choice if you are at the end of a temporary contract trying to finish your PhD or project. I have seen people in the final phase of their PhD thesis writing, who were going to be ditched, from further funding, still getting additional teaching and research assignments in their last months without getting tangible perspectives on an extension. Others got so many additional duties that they voluntary prefered to finish their PhD on social/unemployment funding, or during the evenings and weekends during a new job, because it would give them more time to do so than in their academic job. Some finished successfully, but a considerable fraction did not submit and did not obtain their PhD. Some had to take strategic holidays during the last months of their university contract, just to get enough time to finish their thesis, some were even not allowed to take their holidays. These experiences at horror supervisors were hopefully exceptional from a global perspective, but not from a local perspective. Unfortunately, these things do happen. Is it professional abuse, or is it normal, or both?
Saying no to such additional tasks is a difficult topic to speak about. Because it is very common that supervisors and principle investigators are asking for extra work, without offering corresponding extensions, or additional resources. Maybe the supervisors were once treated the same, maybe they got used to treating others like that. And hey they are your supervisor, at least in Germany they have a big finger deciding in when you can submit and the grading of the thesis, if you get another extension, and possibly they are paying you now. So how much freedom do you really have to say no? Are the consequences of saying no worse, than the work that you could avoid by saying no? If it is unjust, To who in the organization, could you report the situation, ask for counsel, or escalate the situation, without the risk of unfavorable consequences?
Maybe sometimes saying no at work, becomes dangerously similar as saying no in another MeToo type of situations, in which it is often about the power balance and distribution. One of the issues is that in many European universities there is no feedback mechanism or leverage to deal with such "gray area" behavior of Professors.
As in many cases the supervisors (or other people with authority) that give their 'subordinates' the true freedom and possibility to say 'no', are relatively rare. Often the people and good supervisors, who do give the possibility to say no, are also the ones to which saying yes will be a great experience. In the other case, if you have no real choice you probably also have to say yes anyway.