Ela, I am sure that all of that is true, however in the instance that we are discussing, the particular person has not done any of that according to Ratie, so, in this particular instance, I think she has been overly cautious.
Managing a team of volunteers requires tact and the ability to ensure that every person making an offer is made to feel welcome even if, on reflection, it turns out they will be of little use to the organisation. However they should not be left feeling the way Ratie was feeling when she posted - that is, rebuffed and unwanted. This approach can be equally damaging to the reputation of the organisation as someone left feeling like that will discuss it with their friends and a whole raft of potential volunteers may not bother in the future as they 'know' that their offers will be rejected or treated with a lack of the appreciation that every volunteer needs to feel unless they are out and out masochists.
I have considerable experience in managing volunteers so I am aware of the pitfalls, however it is a balancing act which has apparently not been achieved very effectively in this particular point in case.