I don't know how things are run in the UK, I live in France, but if a letter like that was sent by our school, parents would take it as read that they are ALL the PTA.
And then the honest ones would be admitting that unless they have volunteered and organised and given up time, that they are grateful that others do.
Our PTA is struggling and may not continue because not enough people help. Most parents come to events and spend money, which is great and the point of the whole thing. But there would be no events to come to if some people didn't give up lots of effort, time and generosity to making those events happen.
We are currently getting together a questionnaire asking parents if they would rather just get their cheque books out at the beginning of the year and pay upfront rather than buy things at events because we don't have enough people who actually help with the organization.
Which seems a shame because the kids love the events and they really do help all families get the most out of school, regardless of their personal budget.
I guess saying the above makes me a PTA martyr. Something I never thought I would be but which seems to be an inevitability of being an active PTA member because it really is a thankless task. I mean other than being glad that your child and every other child in the school benefits from PTA efforts.
OP you are being petty IMO. Get more involved in the PTA if you want thanks and glory .