Anyone charging for a demo isn't covered on the club insurance - it would only allow for reasonable expenses (travel & materials for example).
It isn't a question of "fairness" and insurance is not mandatory but with no insurance, the demonstrator risks all the assets they own in the event of a claim and for a standard club set up as an association, the club officers are financially liable for any debts incurred by the club including claims for negligence.
It's up to the club to assess the risk and if they think necessary, ensure the demonstrator carries appropriate insurance. If the demonstrator was just there to do a talk, the club officers may decide the risk is negligible so there would be no need for insurance but it's up to the club to assess and decide.
The insurance policies also state that risk assessments must be carried out. Without that, the insurers may decide not to cover a claim and you're back to square one so both clubs and demonstrators should do risk assessments and decide what insurance is appropriate.