I can understand it’s upsetting, if not asked before…but I don’t think this would be seen as discrimination, it’s really for the benefit of people requiring adjustments.
A blue badge is essentially just an accessible parking permit. For wider facilities, supporting evidence can be requested, to ensure the correct facilities are allocated to the people who need them and the legally-required reasonable adjustments can be made.
At a quick Google of other festivals’ accessibility arrangements…it looks like Download and the BBC Radio Two in the Park asks for free registration with a scheme called Nimbus Access Card, which would require similar supporting evidence, Green Man asks for PIP, DLA or GP evidence for certain accessible arrangements.

