It's not acceptable to speak in a rude manner to school visitors.Being generous, perhaps the teacher mentioned in the OP had been dealing with a particularly difficult classroom situation and was more abrupt than she or he should have been. Perhaps the teacher had been told that the volunteers would arrive at X time, and needed to finish a particular lesson before beginning the craft activity, and should have explained that better than they did.
We're fortunate to have a PTA with members who are well-organised and put the best interests of the children first; many of them support the school at other events and trips which need parent helpers. We're very grateful for that, we try to support the activities as much as possible, and we show our appreciation as best we can.
It's a bit unfair to criticise teachers for failing to whoop with joy at the prospect of giving up a Saturday for the school fair/disco/ other PTA event. Teachers generally have no influence over the dates on which events are held, and the events are often held at a busy time of year- summer fairs, for example, generally coincide with the end-of-year data analysis and report-writing which teachers have to fit into their own time, alongside the usual lesson planning, preparation and marking. And not all teachers have partners/ other family members who can look after their own children/ take their children to sports / activities while they go into work for 6 hours on a Saturday.
It's not usually a one-off in terms of extra hours either- those organising the school fair may have seen that Mrs Bloggs from year 6 only turned up for 3 hours to help on the cake stall. They won't have seen Mrs Bloggs arriving at 7 am each morning that week to ensure that arrangements for SATS are in place, with rooms set up according to regulations and enough staffing in place. They won't have seen the teachers who took a residential visit that week, and spent most of each night dealing with children who were ill or homesick. They won't see that the teachers then have to spend the Saturday evening after the fair, plus the Sunday, analysing data, writing a report for the head, and producing action plans for the children who are not on track. They won't see the teachers spending the next week teaching with evenings spent planning, marking, and in staff meetings, followed by a weekend of writing reports, followed by a week of teaching/ marking/ planning/ parents evenings... and so it goes on.
It's not that teachers are ungrateful, it's more the case that fundraising events are among the hundreds of work-related requirements that teachers are trying to squeeze into their unpaid time. That's never an excuse to be rude to the PTA, of course, but it might explain why teachers are not necessarily the workshy fops that some people have implied they are.