For pro bono work we do not charge [but may expect any out of pocket expenses to be reimbursed] on the basis that you agree to indemnify us for any possible loss you or your clients you represent might suffer as a result of any possible action or negligence of ours.
Basically, even though we give our advice, say on the telephone as a result of a casual enquiry, free of charge, we are still liable for that advice. We cannot in law enter into a contract for you not to sue us so the contract is for you to indemnify us. This means you can still sue is but you pay all legal costs including any costs that you win against us. This makes it impracticable for most people to litigate. Blame our American cousins for this sad state of affairs!