I am not really sure that I understand the question.
Who is the"we" that are being told that they cannot perform collective voluntary work and who is doing the telling. Like most sites ours has a set of rules and regulations based on the Local Authority rules. But anyone can get a copy and read them, so no-one can arbitrarily say "you can't do this or that", because people would immediately say "where is that in the rules".
Major works would be an LA matter anyway, but a little bit of group work is commonplace, like path maintenance, clearing a patch of bramble etc. One very generous plot holder dug another's plot when they were ill. Purely voluntary. How could that be against any rules?