Oh it does cost! You are absolutely right there.
but large academy trusts can likely negotiate discounts for signing up multiple schools, so individual primary school teachers and even headteachers/governors might not know much about the scheme nor have signed off on the budget for it.
it only takes a couple of really enthusiastic staff members to get entire orgs signed up to these things - beware of volunteers, basically.
Up until recently Stonewall had such a good reputation that most busy people wouldn’t have questioned it if a colleague had said ‘don’t worry, just say yes and I’ll take on all the legwork’
Some schools will leave dusty old posters up having walked past so many times they’ve ceased to notice them, which is why it’s worth checking the date on any award certificates on display.
my daughter’s secondary was a Stonewall Bronze but it’s lapsed now and it’s a lot easier to go in saying ‘hey, I noticed you had signed up to x in the past - I hope you’ve seen the recent news and won’t be renewing’ than it is to convince a still enthusiastic, currently participating trubeliever.
Both situations should be addressed but gathering info up first and tailoring your strategy is usually worthwhile!