"It is absolutely standard and prudent for an organisation which owns or operates a facility to require someone who wants to the use that facility to have a prudent level of insurance for third party liability. It is likely to be a requirement of their own public liability insurer."
All I can add is that worldwide this is becoming more and more the norm - the rise of legal firms on a no win no fee basis means everyone reaches for the lawyers if anything happens - and they go looking for people to sue. The requirements are often buried deep in the T&C's you sign up for.