I'd like to think this wouldn't happen at our school, as I've seen my son off on 3 school trips so far and they tick them off at the school gate before they are allowed to get on the bus, so any extras trying to sneak on would be stopped at the school gate, not get anywhere near the bus. Also, but it is a small school, our teachers know all the students by sight and name, so would know if someone shouldn't be there because they're not on the list.
Re. the cost of the trip - same in Australia, we have 4 residential trips that the students can do, in years 3 - 6 - they're great for the kids, really good for confidence building etc. Yes, they're expensive (depending on distance and time away) but, like in the UK, there are hardship funds and payment plans available. Most manage to go - the ones who don't are usually those with entirely disengaged families (we have a few of those, low socio-economic demographic area) or the ones who actively do not want to go because of anxiety or other issues. Even then, these children are supported and encouraged to go wherever possible, and most do manage it.
I do think the parents were right to address the school's safeguarding but I don't think they were at all right to take it to the press. So to some extent I do think there was an element of self-promotion involved - but I wouldn't want to expose my DD's atrocious behaviour to the world like that! I'd be mortified!