Wotnochocs, did ds return the reply slip?
If it's just for non-payment and the slip's returned, that's certainly NOT on, but if it's disorganisation / hasn't bothered to return the reply slip as he doesn't fancy it, then the threat of detention might be legitimate.
A lot of 15 yr old boys don't fancy a trip to see some poets
The poetry section makes up 50% of the lit exam and a 25% of the language exam and Poetry Live can directly impact a student's results because the poets and the Chief examiner are there to give their advice.
You could look at it the other way and legitimately complain if a school did not offer activities/trips as a disservice to pupils' education. Schools try very hard not to just cram-to-exam but give the students a broader, enriching experience. The problem is that Arts Council funding is disappearing so there will be fewer visiting theatre groups, for instance, school's music provision is under threat so individual music lessons, which were part subsidised, will increase if the school is to continue to offer them etc etc. So it looks like hard-pressed parents might be asked to contribute more (and increasingly can't pay) or just costed out of the curriculum 
The wording of "voluntary contribution" is a requirement by law, but is misleading as you can't exclude a student on the basis of financial hardship, but essentially it is a pay up letter.
If ds doesn't want to go, then alternative provision will be made, although it might be poetry Q&A!